It's tough to depower the main on a dead run, but it's possible to a degree by sheeting the main IN and presenting less area to the breeze. This can be difficult to do if it's windy, and is inherently a bit risky because your are putting yourself closer to an accidental jibe.
If you find yourself overpowered on the run, you're best to come up onto a reach and reef (if you haven't done so already) or douse the main and then run on the headsail alone. Hopefully you'll have the sea room to do so.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)