If it's a guitar that uses a pickguard, like a Strat, then just get a pg that only has the holes you want. My Strat, for example, only has a bridge humbucker, the other two pickup holes are covered by the pg. If the guitar does not use a pg the best suggestion I have is to get a cheap pickup to put in the hole as a dummy pickup. Anything else is going to look ghetto IMO.