Dairy products cool off hot spices, so maybe a layer of cheese mixed with sour cream would do it. I haven't tried this, though, so I don't know. I made a sort of Mexican lasagna for a party last month and it seemed a bit hot to me - I provided a bowl of sour cream right next to the casserole pan for those who wanted it.
Or you can dilute the salsa by mixing it with a mild version.
The trouble with eating Italian food is that five or six days later you're hungry again. ~ George Miller