Delete the .maintenance file at the root too. I had the same problem, even after I removed the plugin from the content folder. Fortunately, getting rid of the .maintenance fixed everything (other than the fact that I still don’t have a functioning buddypress).
I got the same thing, and was about to restore from backup when I discovered that removing the .maintenance file at the root completely reversed the damage from activation.
Hope you find the same applies to you (it saved me hours of work).
