I bumped into a parking pole last night and the plastic on my bumper popped right back out, but the metal caved in about 4-5mm. Nothing major but it was too much for me. So I pounded it out with a hammer from the inside, it looks perfect now, but a circle of spider cracks, about a quarter size, appeared in the clear coat from pounding it out.
What can I do so they do not spread?

 

Get a TJM or an ARB!

You might could get it sanded, primed and repainted, but for what that would cost you could possibly do the above as they'll probably want to paint the whole bumper etc.

Bummer dude! and Good luck!