The way I unlinked was:
1) Account Settings ->
2) Overview ->
3) Right next to your avatar there should be a link to disconnect G+ from your youtube profile.
Thanks a lot! It was "Youtube Settings" for me, but that may depend on the exact route one takes.
If you recently joined, you can't. I just went through this yet again, and they now have a "14 day period" where you can unlink. After that, as far as I can tell, it's permanent.