Ummm ... what is it you're trying to do? I know what it sounds like, but that can't be right. Nobody would buy the fastest mainstream CPU available, and then want to make it slower.
Perhaps more importantly: why do you want to do it?
Who knows? We certainly do not. I can understand building a computer no more powerful than you need, an HTPC for example. But in that case, you do not spec a top line CPU.