How to disable Skype's Incoming Call popup on Mac?

I've searched everywhere for an answer to this but it seems there is none.

I'm using Skype (5.1.0.935) on a Mac (Mac OS X 10.6.8), and there is one thing that is driving me mad. When someone calls, an "Incoming Call" popup appears, interrupting whatever I'm doing.

Is there a way to disable this? There is nothing on Skype Preferences, I also tried editing the config.xml file, adding a 0 and 0 which apparently works on Windows, but nothing.