An external marketeer has worked on a project throughout collaborating with JIRA. However, his job is finished, but we are still working with the same JIRA board. He still gets updates etc. How can I delete this user from my JIRA board, or at least deactivate him?
You can do both but my suggestion is to deactivate it, you can delete it after some time.
Go Jira Administration -> User management -> find user -> edit
There you can uncheck "active" user will no longer have access to Jira or receive notifications.
You can press tree dots next to edit, there you can delete the user.
Documentation on user managment
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