You can delete a user from your Gitlab organization by going to
Settings and selecting
People. You can find the list of your organization users here. You can filter this list by name or by user permission.
If you do not find the user on this list, please make sure to check the
Pending list as well, in case the user has not accepted the invite to join the organization yet.
Once you found the user, you can click on the three dots at the end of the user's row and select "Remove From Organization". This will delete the user from the organization and also remove any additional project-level permissions for the user.
Please note that the organization is required to have at least one admin user, so you will not be able to delete the last admin user.