I've seen some work on this, but it's a long way off before it gets into humans. I think it's only been done in bacteria or simple multicellular organisms, but more complex organisms like humans have a lot of barriers before this works... It's very easy for this to screw up, and it's very difficult to do. You'd have to be certain that it will cut just that DNA region and ligate it back together correctly. Also, there may be issues with chromatin packaging and correctly recognizing the site. It is at the very least 15 years away, and even then you'd have to deal with the FDA being super cautious.
It would probably be easier and safer to insert the genes necessary to tag the protein product of a specific allele for destruction right away, or to use something like RNAi to destroy it right after transcription.