You can exclude tags in a search so if you don't want any crossovers or Steve/Tony at all, you can filter it out. You can also filter your C/N results by F/M -- that should get rid of the fics where a slash ship is the primary one. It'll also take out any fics tagged Multi, so eliminate any ensermble/multiship fics if that's a concern, but if you're just looking for c/n alone, that's probably the easiest way to narrow it down.