As Spam is a real big Problem within the Internet, today nearly no one gets around it, as Internet consumer(someone who doesn’t offer own communication platforms) you may not notice that Problem too much, maybe you didn’t even noticed it really. But it’s definitely a real Problem, Spam is everywhere.
You get it with your Daily Emails, already when you input your Email once to some untrustworthy Mailing list or application, and from that date it never will stop again. If you’re lucky that’s everything where you get into contact with Spam, as soon as you own some Blog/Forum aso. your Spam contact will be much bigger. How can I now prevent these platforms from Spam?

Registered Users only

One Option which is widely used is to allow Postings only to registered Users. This keeps out every Spambot who doesn’t have a routine to register to this platform(or to register in general). Additional many registering processes require that the email is validated, if that doesn’t happen the account will not be able to use this Account. Again that will prevent many Spambots from Posting Spam, as many Bots do have some registering routine, but they don’t use valid Email Accounts and/or don’t do the needed steps to activate the platform account.

What’s the reason that they don’t do this?

Mostly it’s the reason that there are enough platforms outside who still doesn’t use such a protection mechanism. Sadly that method may take away some Users, who would like to post some Comment/Post, but they’re not willed to create therefore some Useraccount just for one or two single contributions. The reason why they’re not willed to do so is,(you will know it mostly as you’ll surely think similar) is that they’re afraid of getting Spammed on their Email. And that only because he registered at some small website who delivers your Email to some Spam mailer.
Fortunately there’s a way to receive Contributions from non-Members, and that without a big level of Spam you’ll have to fight with.

Spamblock and -labeling

One reason wasn’t mentioned above explicit, but you could read it between the lines. Maybe you, the platform owner, want although to catch the users who don’t want to register at your site just for a single post. But this position don’t needs to be negative for you. There are many solutions out who are really nice and widely used, so proved to work well.
In general I see three different kinds of applications, who mostly differ between what needs to be done in future by yours:

Install and care yourself about

These kinds of Software is just installed, then the User needs to add what words will block the user contribution, that kind of blocking was more commonly in the early days of spam, where no services where available to Check Messages for Spam. You have to take care of the filter list, as the spam messages get changed regularly. At some point you’re within a dead end, the spam messages doesn’t contain any words you can block without blocking potentially legit posts. This kind of App, isn’t doing too well any more as Spam gets changed regularly, and sometimes don’t even appear a human to be Spam on the first moment. The mentioned problem is more likely to happen on Websites which handle common Spam themes like Real Estate, Pharmacy and so on.
An example Can be get here for phpBB

Human or Bot Tests

Some quite well working approach is to decide upon a Message, is Spam or not, is to ask for something which is only doable by a human. The best example is the CAPTCHA, the user is asked to type in some Letters and Numbers from a picture in order to be able to post the Message(or tagging it as no-Spam). But you need to be careful, there are already some Bots out who can read early CAPTCHAs or weak ones. On the other hand there are newer CAPTCHAs out which are quite hard to solve even for no handy caped people(just imagine how hard it would be for handy caped ones). Some good CAPTCHA method was developed by Microsoft you’re given 9 Photos of Dogs and Cats and you have to select only the Cats/or Dog ones. The project is called Asirra (Animal Species Image Recognition for Restricting Access) and is powered with photos by Petfinder.com
Then you have some Checkboxes you need to click if you’re a human(this is nowadays no problem for Bots any more, as they fill in every field). Another idea is to ask to calculate two values together through some addition, subtraction. This one is quite hard to solve for them, a newer version of it is to ask some questions which needs to be answered.

Spamlabeling as Service

Nowadays you’re served with Webservices who check your Email/Comments. These are used by many users and do catch the latest Spam quite fast, every User is able to improve the Ruleset as he can report false positives or not caught Spam. The best Example would be the well known Akismet.
These services are mostly the future of Spamblocking as your work is quite low at all, and you have a real good rate of false positives to catched Spam.

Conclusion

Spam is a really heavy Problem, since some years and will mostly stay another few years as long as no worldwide antispam act is done, so the Spammer can be suit everywhere one the world. But you have since the start some really good Protect mechanism, and fighting Spam was never easier before with Services like Akismet. Don’t belong to the People who loose time and money through Spam. Fight it, you’ll not regret it!