What type of ch system have you got? Vented, combi, etc? Are other rads rusty like this?
Often, badly rusted rads are caused by pumping over. If, when your system is on, you have a small loft tank with vent pipe over that is running, this could oxygenate your system's water. This causes rads to rust out quickly. If this is the case, it needs sorting ASAP.
And, as above, the only sensible thing to do with the rotten rad is leave it out for the p!keys.