P38 Drag Link ball joints

Hi,
I'm in the middle of replacing the drag link and appear to have a problem tightening up the ball joint nuts. The nuts are Nyloc and the torque required to run them up causes the ball joint to rotate, preventing the nut from tightening further.
I've just figured that I can probably use a trolley jack on the drivers side to press the ball joint up into it's taper in the steering arm, preventing it from rotating.
However, on the passenger side where the ball joint is above the arm, the stud points down and the nut goes on the bottom, I wouldn't be able to use the trolley jack.
Any advice would be appreciated, as I'm starting on the tie-rods tomorrow...

Make sure the tapers are clean and free of grease (I know - it's tempting to use Coppaslip!) I've used a crowbar against the chassis to lever down on track rods before now - or even an old track rod!

Thanks for the response everyone; Neither the mallet nor large hammer could get the ball joints to stay in their (clean) tapers, however the trolley jack worked on the drivers side and the jack handle used as a lever worked on the passenger side. Job done.
The track rods were much easier, especially when i discovered the internal torx on the new ball joint studs which prevent the joint moving (Thanks BritParts). Doh! didn't notice them on the drag link, that will teach me to try fitting stuff in the dark!
Rear Brake pads to fit tomorrow - bring it on!