The "can't use" part of the power which effects opposing characters. Is this countering a power like outwit would counter or is it it's own terminology which stops opposing figures using the specified power/ability?

If I were to choose willpower, would this stop figures with the cosmic team ability using willpower?

Also does the effect of not being able to use a power/ability by opposing figures count against all sources of where that power could come from?

It is not countering anything, so it would block the use of WP on a PC character.

It does not matter the source, if you can't use it, you can't use it.

Cheers. This is what I thought but could anyone give me a link to an official ruling on it please?

Page 27 has a break down of the difference between counter and ignore.
"Can't use" is simply a third term. The same principle applies.