mobile input / OnMouseDown / multi touch (most likely)

So I've been using OnMouseDown to test my game while I've created it. It does work fine for mobile, since I really don't need more then one input (I'm choosing a character with a UI button, and then clicking a collider to tell the char. where to spawn). The problem is sometimes when I'm holding the phone my thumb will touch the screen, which does not allow the OnMouseDown to register anymore.

The multi touch posts I've seen don't make much sense to me, and the best that I could get working was a multi touch that registered all my UI buttons at once, and didn't use the colliders (just spawned from the same collider).

is there a way to stop certain areas of the screen from registering input? if not how do I get the touch controls to know which button/collider I am touching?