limit the rate of posts they can make to one per 10 minutes, or something similar?
That's a good idea. It's not possible for a human to create 2 or more meaningful topics in 1 minute.
Only English charactes should be allowed in a topic title, it's an English forum after all (analphabet A-Z plus basic punctuation marks - period, question/exclamation mark, brackets, dash, apostrophe - what else a normal user needs).
All those restrictions could be called off for a trusted member. Someone with account's age greater than a week/month and with 10+ posts.