Go through the following steps to deactivate a user:

1. Navigate to Administration > User management > Users.
2. Click on the Edit link for the user that is to be deactivated.
3. Uncheck the Active option.
4. Click on the Update button to deactivate the user.

Deactivated users will not be able to log in to Jira, and will have the Inactive option displayed next to their name.