It can deny the use of it's services to people using ad-blockers.
It's possible that we see a migration from youtube. Maybe some people will be ok with migrating from gmail (probably not the majority though. I did it and it was HARD). But very few people will stop using the search engine or google map, because it has absolutely zero decent competitor. I tried them all for months, fairly. Their are not even on the same planet.
So apart from a few geeks, people will just disable the ad blocking software if google decide to force their hand.
Besides, wait for the W3C DRM standard + webassembly to be used to hack very hard to block ads...