When someone adds a reply to a topic, can his Reply button on that topic be disabled? So he can use Edit instead. And can this feature be enabled or disabled and every group can have permission to bypass the option or not??

This can be done presently by the admin selecting a longer time for automatic merging of consecutive posts.

yes but this would be much better...

Your method would also render the time limit I have presently on editing useless.

Well, you can just have the post handler check if the last poster in the topic is the same user trying to submit a new post and then return an error as necessary. Of course, delayed inserts or concurrent inserts would still get through.