Mikrotik should have a feature like this.
http://denyhosts.sourceforge.net/
There is also this:
http://www.hexten.net/pam_abl/
Matt
I think the solutions mentioned in the first posts of this topic solve 99% of the problems: http://forum.mikrotik.com/t/limit-bogus-ssh-logins-solved-at-least-for-me/10358/1