I'll add in a button to remove a save game from the list.
I don't really get the problem, though? Oh, is it because you've opened so many maps it's pushed your current game off-screen? That sucks, m8, sorry! I don't even know of a manual fix apart from deleting your "opt.txt" file in the %appdata%local/HOMM_tiles_mapgen, which will clear all your saves.
There's space for five maps at least, and at least by the next update, it'll be the five last-played games. If everything works as I want it to, you should never have to worry about save slots. Sorry for the temporary inconvenience.