Try a small vise-grip pliers- lock them on and tap that with a hammer.
I've had these put up a fight before but usually just using a little impact to break them loose does the trick.

Also, of course rotating the engine a bit so you can get a better bite will help. Just turn it to get a favorable angle on each one.

You'll need new M6 nuts once the old ones are off. The studs should come out with the vise-grips as well. Obviously avoid damaging them, as the new water pump does not come with any studs.
--
Rob Bareiss, New London CT ::: '87 244DL/M47- 234K, '82 245T/M46-182K, '84 242DL/AW70-100K, '89 244DL/AW70- 212K and I miss my: 86 244DL 215K, 87 244DL 239K, 88 744GLE 233K, 88 244GL 147K, 91 244 183K