Log out and then log back in a second time on the same computer.
This will reset the last login date and will allow the mobile account to be removed once the expiration time is reached.
The last login date is recorded in the /var/db/shadow/hash/uuid.state file. After the first login on a client this date may be set to an incorrect time. For example:
Logging in a second time on the same computer will properly set the last login date and allow the account to be removed once the expiration time is reached.