As takrl says, there's no good way of preventing this beyond the review system that is already in place. We don't want to prevent askers from suggesting edits to answers because there might be a typo or other minor bug that they can easily fix to improve the answer. Half of the time, the person who posted the answer will accept this edit themselves.
So when you see edits like this, make sure to reject them, just as you have been doing.
If you'd like to "go the extra mile", you can also visit the question and leave the asker a courtesy comment. Something along the lines of:
I noticed you tried to edit one of the answers to your question to ask for clarification. That's not really the way that Stack Overflow is intended to work. If you'd like to ask a follow-up question, you should either leave a comment to the answer (you can always comment on answers to your own questions, regardless of reputation) and/or edit your own question to include additional information.
This can be an effective way to guide them in the proper direction without requiring any additional features to be implemented.