Ah, the nature of all things, to rot and decay. Gotta love entropy. Well, there's still no excuse not to fight against it though, and this is one way to do that.
Paint your head (your machine's wheel head to be exact).
Get a good spray paint (Rust-o-leum makes some really nice epoxy spray paints in some very cool colors- hey live a little), turn your wheel head on just long enough to get the wheel spinning and paint it from center to edge with a nice even coating. The nice even coating is critical, don't make new bumps and ridges.
Of course if you do already have rust on your wheel head, then you're going to have to remove it first (nothing for that but elbow grease, sand paper, wire brushes, scotch brite pads, acetone and varnish- among other things).
When your paint job starts to wear through, just strip off the old paint (this will be much easier than removing the previous rust) and re-paint it again. The frequency will depend on the wear and tear on the wheel head's day to day use.
Of course, if you've had rust on your wheel head for a while, then most likely it has transferred to the back of your diamond disk (it is magnetic after all). No problem, just clean off the back of your disk also.
To do this, just use a scotch brite and some acetone. Pour some acetone on the magnet side of the disk and use the scotch brite pad to scoure the rust off the magnet. You'll want a spare clothe handy to wipe away all debris as you're working on the back.
We suggest taking it a section at a time. Work it until the back of your disk is nice and clean again and everything will start working nicely once more.
Above all, don't store your disks on the machine. Take them off your wheel head to store them and allow the machine to dry between each use.