One great way to tone it down without changing the flavor much (other than more avocado) is to add sour cream. I never intentionally add sour cream to my guacamole directly: I like to taste the avocado. But, it's pretty common to have it on the same taco as the guacamole, so this is actually a time-saver (like shampoo and conditioner in the same bottle, but tastier). Yogurt would be another option.
Another tried-and-true way to counteract heat is with SWEET. This will change the flavor, but adding honey will also reduce the heat. To be honest, I wouldn't do this in a guacamole, but it's something I've done with Asian food successfully.