Of course the only perfect way is not to put it on the web.

A long essay can be written here but the basics are:

registration
moderation
ban the abusers

If you want to permit postings to run on auto pilot with purely mechanical checks such as captchas then you will get abuse.

you can also add a time limit for each thread. If it turns out that it is an interesting one then you would keep it open for a longer period of time.