The Remove button is greyed out in my BOINC MANAGER Projects display. And, I'm getting notices that for some reason I am logged in twice to NFS@Home. The notice says I need to remove the project. But how?
Curious.

If it's greyed out in BOINC Manager, then you joined that project via one of the other Managers.
Look in the Menu under Tools > Use Account manager, and see if any of them look familiar.

You have to go to the website of the account manager and do it from there.

That's what I've been trying to do. I go to the website for each project I want to remove, but there's no obvious way to do that. In particular, the NFS@home and the Citizen Science Grid. Both are giving me notices that I need to remove the project and add it back in. Citizen Science says there something wrong with my key, and NFS says I'm attached to the project twice.

But how?

Have you found out which Account Manager you used?

That's what I've been trying to do. I go to the website for each project I want to remove, but there's no obvious way to do that. In particular, the NFS@home and the Citizen Science Grid. Both are giving me notices that I need to remove the project and add it back in. Citizen Science says there something wrong with my key, and NFS says I'm attached to the project twice.

But how?

Not the website of the project but the website of the account manager - something like BAM or Science United - that you used to attach to the project in the first place.