There are two basic methods:
1) the quick method - attempt to flush out the nut with a solvent/oil - something like WD-40, mineral spirits or kerosene. Soak the entrance of the nut and then run the nut over the screw, repeat many, many times. There may be wipers on the ends of the nuts - if they can be removed or at least loosened up, this would help with this purging procedure. WHen you are done, the nut will need to be lubricated properly. Do not attempt to clean out the nut by pumping rediculous amounts of grease into the nut and make sure you look into the correct type of grease to use - don't just use whatever you have. Get the right stuff and don't over-do it.
2) the correct method - take the nut off and clean everything really well and then re-assemble - it's not all that hard - you just have to be careful not to loose any of the balls. The disassembly/re-assembly procedures depend on what kind of nut you have. Posting a pic might help generate more replies to this thread as that would clarify what style of ball nut you are working with.
Scott
Consistency is a good thing....unless you're consistently an idiot.