While doing deployments, we have a number of virtual machines that we use as copies of customer production environment; for testing; learning etc. We installed one such machine using a trial key from Microsoft site. As key is only valid for 90 days, it expired, but we still wanted to use that VM for demonstration.
We applied partner MSDN key instead of trial key but once we did that CRM informed us that we reached user limit.

User limit
You have reached the maximum number of users.

It turns out that our partner key supports only 10 users, and we had about 25 users in that system. Since you cannot access the CRM interface we had to play with database a little bit to disable the extra users.

The following procedure is probably not supported!!!

You can administer user accounts directly in CRM database. Users are located in table SystemUserBase. There are two columns that control status for each user:
IsDisabled (bool) – should be True for all disabled users
AccessMode (int) – 0 for full access; 1 for administrative access; 2 for read-only access

Modify these to match your license and your CRM will be operational again…