There's a discussion on this with some code here: https://community.marketo.com/MarketoDiscussionDetail?id=90650000000Ppy4AAC
When I had issues with crawlers, I had my webmaster code in blocks to different fields. She was able to block things like 1234, None, and bogus characters like that. I don't have the code, but I do know it's possible, esp with forms 2.0 if this your first go-around with them.