Insulating a via during reflow?
You want to "tent" your vias. This means changing your DRC such that vias under a certain drill size will have solder mask extend over them.

If you are using Eagle, this is accomplished by changing the "Limit" value on the Masks tab of the DRC. The limit specifies what size drill hit that the solder mask should cover up to.

Check out the tutorial from Sparkfun on better PCB's.