Bigger hammer, or an air hammer .
If you have a large brass hammer, try that before moving up in scale of 'weapons' !!!!!
Don't even think about trying to pry it off !!!!!!
Back the nuts off a thread or two before you start 'beating' it into submission !!!!

OK now seriously, depends on how much time you want to spend on this, you could try leaving the nuts loose a few threads and see if it will loosen up on it's own in a few days .
Other than that a bit of impact persuasion is going to be necessary on the on the pressure plate .
You don't want to get too crazy with impact use, as you are indirectly beating on the crankshaft .