You may want to make another batch of the cooked salsa, but leave out the peppers--combine the two batches. Salsa keeps for quite a while because of the acid content, and you can also store it in the freezer (it will get a little more watery once thawed) if you have too much.
You could also try to just add more tomatoes and onions as others have said, but I think for flavor you may be better off with the doubling. It really depends on how much extra heat you ended up with (a little extra will be solved by a little tomato, but not way to much heat)
Good luck and happy munching!