Well how long can you be without your bike???

I would start by simply plugging the hole where the stem bolt goes in and turning the bike upside down and pouring some non-Solvent based light weight poser oil (WD-40) down the hole between the forks - Now you just Forget About It (this is the hard part)...

If that fails after a few days or weeks then I would try something else down the tube always being careful about what can effect the paint...

The list is long: PB-Blaster, Diesel, Vinegar, CLR....

Don't worry man, unless its been abused its going to come out OK...