give it a good clean out with salt and oil, or use a Brillo pad and water, then rinse and dry. You won't get all the rust off, but it should be fine. Then heat it up over a hot flame with some more oil and salt in it til it's smoking hot (not loads, just enough to make an oily film). then, turn off the heat, protect your hand with an oven glove or something heatproof, and wipe out the second lot of oil and salt with kitchen paper. The Paella pan should be fine for use. I do this with mine if i haven't used it for a while - they tend to go rusty anyway.