Q. I'm trying to delete a user, but the option is grayed out - how can I do this?
A. If a user has any work associated with their user name then you can't unless you remove them from projects and activities or delete projects and activities along with time logs associated with them.
You can just make a user inactive instead, that will hide them from all lists and free up a seat in the license:
Pic. 1. Making a user inactive