The first project that I crunched a long time ago was SETI, but nowadays I prefer other projects. Therefore I haven't chosen SETI as one of my projects in BAM. The problem is that one of my hosts still continues to be connected to SETI. The button in the boincmgr for detaching from the project is greyed out (for all projects, possibly because that should be handled from bam) and since I'm not connected to SETI in bam I cannot detach the host using bam either.
Any ideas?

Have you tried "Reset Project"?

The SETI servers were down for a while but once they were up again I could attach bam to SETI. Then I could let it detach again and now my computer also detached. Problem solved! Thanks!