You can't turn it off. If you could, that would be bad. Real bad. Like, PC-go-broke, bad.
Anyway, what's on that partition? If it's your primary partition (the one with the OS on it) then you can happily be assured that the space is getting sucked up by your page-file and unseeable Windows system files.
Otherwise, I think it's time for a scan, defrag, and maybe even a reformat.
One thing you could try, though, is to clear out that partition. If you have nothing on that partition but junk or archived stuff, you can just move it (or back it up to CD or something) then use the disk manager to reformat that partition, clearing it all out.