Eric Before doing ANYTHING, I would first try to screw it as far as possible INTO the dropout... and then back it out again. That way you'll know for sure that it is not frozen into the dropout with rust. If it is, you should proceed very cautiously so as not to either break off the bolt sticking out the back, or bust up the slotted end so a screw driver will no longer hold. If you lose either end, you'll severely limit your options. If it turns okay...
Cut off the screw with a small "cut-off wheel" on a Dremel® rotary tool, then back it out with a screwdriver. If you don't already own this tool one of your neighbors probably does. ~ Better still: buy one! And do spend the extra few bucks for a cordless model - simply for convenience when using it on any number of bicycle related projects.
My skills with even very fine hacksaw blades are simply awful. The tiny (1" diameter) cut-off wheels will make an absolutely clean cut with no need for repairing damaged threads on a bolt end.
I now use these all the time when replacing rusted bolts with new stainless steel ones and want to trim down the new bolts so they'll fit perfectly. I've even used this for cutting off frozen binder bolts on stems when either the bolts have rusted tight onto the nuts or when the concealed "keys" beneath the bolt head are so damaged that the bolt simply spins when the nut is turned. ~ if these little discs can cut through an 8mm bolt, they will make very quick work of a 3mm adjuster screw.
PS: That's not all...
These little marvels are great for cleanly cutting cable housings, too. No need to file down the jagged ends of the spiral wound steel which is typically left when using a diagonal cutter....
And, they're also perfect for cleanly trimming off any excessively long stays on your Bluemels mudguards. Or even trimming down any bolt ends protruding through to the insides of your LeFol or Honjo alloy fenders...
But wait! You can also trim those rack mounting bolts to the lengths you really need for a beautifully flush frame fit...
Okay,... sorry guys. My apologies for sounding like I'm selling a Ginsu knife set; I just get a bit overly excited when I discover a common modern power tool which actually works better than I would have expected.
Good Luck Eric!
Bob Hanson, Albuquerque, New Mexico, USA