(Note: my previous comment mentioned only the sanding method, but I researched this reply a bit and now I think the chemical soak method is probably the better first try if a “normal” cleaning with a kitchen scrubbie or whatever isn’t sufficient.)

Soak the entire pan in acid (vinegar, pool cleaner, acidic drain cleaner – whatever) and then scrub it with a stiff-bristled brush. Make sure every part is completely submerged, handle and all (edit: wait, you said “dutch oven” – if it’s got a wire bail handle, remove that first), 'cause otherwise it can eat away at the pan at the point where the liquid meets the air. The stronger the acid you use, the quicker it’ll work but the more you’ll have to watch it because it’ll start eating away at the metal once it finishes off the rust. Wear safety goggles and gloves, BTW. Alternatively, if the prospect of acid sounds sketchy, apparently Evapo-Rust is safe for cookware, so that’s neat.

Otherwise, if you want to keep trying to remove the rust mechanically, basically try harder and with stronger abrasives. That’s up to and including sandpaper, if necessary.

If the pan is pitted, either use the soaking method to get rid of the rust and just deal with cooking on the uneven surface until the seasoning builds up enough to get it out, or consider resorting to power tools (palm sander, die grinder with abrasive disc, angle grinder with flap disc, etc.) since you’ll have to remove a lot of material to get the surface smooth and flat again.

In any case, once all the rust is gone, wash it with soap and water, dry it off, and season it immediately, before any new rust has a chance to start forming.

Clean and strip seasoning with yellow cap oven cleaner(lye). If any rust remains, use a vinegar bath and scrubbing until it comes off. When you use the vinegar and get all the rust off do not let air dry, wipe dry then heat on stove until liquid evaporates. Reseason.