Removing a User
Before a user can be removed all edits, comments and spaces must be removed or the user will not be removed.
Disabling a User
Disabling a user gives the license back so that another user may be created. There is a known bug that disabling a user will not give back the license due to the user still belonging to a group and or the cache statistics have not been flushed.
Disabling a User bug (Solved) - https://confluence.atlassian.com/display/CONFKB/User+Count+Not+Updated+After+Disabling+or+Removing+a+User