what can you do about a nut that turns on the bolt when trying to get it off?

I assume you have tried holding the nut? You could tack a weld on the nut to the plate...

yes i'm able to hold the nut but no welder!!

Put a spanner on the head of the bolt and another on the nut and twist until it undoes.

I'm guessing that because your asking youve tried this already and you cant get a spanner on the head of the bolt. If this is the case - buy or borrow some nut splitters, ive got some, and use them to remove the nut

http://ww w.screwfix.com/app/sfd/sea/searchresults.jsp?_dyncharset=UTF-8&q=nut+splitter&pn=1&pd=1&pi=1&cn=1&cd=1

i can get a spanner on the head of the bolt but the nut is going round and round but isn't coming off - how effective are the nut splitters?

try levering the nut upwards if you can, as you turn it, it sound as though either the thread is damaged or you have gone past the thread and onto the shoulder of the bolt.

HTH

Ray

Nut splitters can be very effective, however I haven't got one and so use a cold chisel and hammer. Vary size of chisel and hammer to suit nut. i.e. little ones for your little nuts and big b****r and short handled 7lb sledge for the rest. Take care not to bend the bolt/stud if its critical. Make sure the chisel is sharp. Hit it don't tap it. A few good blows are better than lots of gentle ones. best of luck, I know its a pain in the *rse when the thread strips.

woo done, the thread was b*gg**d. i used a chisle and hammer and also a screwdriver and the trusty dremmel! thanks for you help - on with the strip! - OF THE CAR
Rescued attachment nut split.JPG

The bolt
Rescued attachment bolt.JPG

nicely mullered

As a matter of interest what was it supposed to be holding together? Can't imagine it was making a very good job of it!

It was a bolt holding on the gearbox cross member on my sierra!
Rescued attachment busted.JPG