A valid contract online requires mutual consent. A unilateral contract which is a one sided contract has often been found to be unenforceable. There are two types of ways to gain assent: affirmative and implied.
At a minimal you should have a license to the website to the user. If you are under a subscription model you should have the terms and prices easily available. Include API terms as well as some legal language around idea submissions. Perhaps the most important step is to step up rules for your website.
It is important to define what is unacceptable behavior. This normally includes harassment, obscene language, or engagement in illegal activities. You should also prohibit uploading viruses, interference with the network, hacking and disruptive activities.
The minimal number of provisions you should include: disclaimers, limitation of liability, choice of forum, choice of law, severability clause, arbitration, termination, amendment, repeat infringement, and service discontinuance.