The Jester is right that there are more kinds of 'power' for a BBEG than merely PC combat-prowess. And Felon is right that it can be hard to get PCs to respect that power when the BBEG has seriously ticked them off a few times!
Great wealth, strong social position or potent social connections are also sources of power - and ones that the PCs find hard to match and defeat.
Yes, I have used this, in a story involving a nobleman struggling to take over a town, who was also involved in his own little demon cult to extend his life, and who had dealings with the local Thieves' Guild, for whom he did favours. He, himself, was a low level Aristocrat, who ahd gained a couple of Expert levels as part of his cult. But the PCs had a devil of a time getting at him, even after they uncovered a probable connection to an assassination attempt on a more senior noble. He was careful enough to use cut-outs, and the Thieves were protecting a valuable asset in him. In the end, they accepted collecting a lot of circumstantial evidence against the man, and presenting it well to most powerful local nobles. They were quite satisfied when the nobles took matters into their own hands, and the annoying BBEG died when his mansion 'accidentally burned down in the night'!
Less satisfying than killing him themselves? In the end, it was a bit, but they did find it satisfying to be able to tell each other 'we were instrumental in eliminating a demon-worshipping and corrupt pervert'.
So yes, such BBEGs can be used - but you need to consider how power and influence work, and the consequences if the PCs decide to ignore it all anyway.
