When I put our couch up as a maybe (I'm rarely YES as I'm always working). My tip to avoid the crazies? Only respond to personalised couch requests, just as a sign they've made the effort to read your profile (meandmybfhaveonetogether). For us: If the message is all gravy, I'll let them know what's available, how long for, etc (which is usually avaiable on our profile but theres no harm reiterating)
Copy and paste messages send up an alarm with me, and it's an instant no go. They always just get a polite copy and paste decline reply. Also personal messages (these go straight to your inbox) arn't ideal either. I don't know why people can't just like, click the "couch surf with.." button and do it properly, it makes things so much easier, as the personal message folder makes things messy.
As you probably know though, you can always get a weirdo, but generally, they're rare and shouldn't put people off this. It really is such an awesome thing.