Actually I don't believe you need a method to 'filter objectionable material' but you do need a method of deleting objectionable material after it is reported as objectionable. (i.e. you don't need a system to auto-detect that this word - **** - is objectionable and automatically convert it to a bunch of " * "s like this bulltein board can do - but you do need to be able to change this post so the word, or the entire post, is deleted)
How to block a user -
1) assign each user a random number as an ID
2) add that ID to each post by the user
3) if you need to block a user, add their ID to a list on a website that you maintain (check out google.sites for this)
4) before allowing anyone to post, check your website for their ID
You can also do this on CloudKit. If you use CloudKit you can use the user's userRecord ID as their ID. You could also make an entry yourself in the userRecord of an offending user using the CloudKit Dashboard.