In terms of grammar errors and spelling errors (they are not typos) I think it would not cost that much to ask people when you hire them, "Are you a good speller? Can you write?" Or you could hire only people who could demonstrate spelling and grammar skills through a writing sample. I know you won't actually consider these tests which are fundamental at a normal interview, but that is a lame excuse.
Writing is, after all, what they are doing on here. They are not web programmers. Or if you believe that it would slow things down too much (as you say) just don't write anything yourselves. Just put links to other people's writing.