Save games stored on the computer

Here's a quote from the developers:

Q: I changed the in-game resolution, how can I restore the original settings?

A: The game will auto-detect your monitor's proper resolution on start-up. If you change your resolution and the proper one is not available in the options menu, you can have it auto-detect again by deleting your profile here (exact location may vary): "\Program Files\Steam\userdata\33142021\107100\local"

Source

Well, hopefully you'll find a better answer, but one workaround is to simply back up some of your older saves to another directory.

On Windows 7, my saves are stored in

C:\Users\MyUserName\Documents\BioWare\Mass Effect 2\Save\John_12_Soldier_260110

That last bit differs on a character-by-character basis. You could grab the .pcsav files from there and move them to a backup directory. Now you've got some archived saves and free slots. If you want to restore them later, you just drop them back in (though you probably need to make sure you aren't bringing the total over 50 if you do).