When it comes to Deactivating a User you can do so as an Admin in RiTA via navigating to the Admin navigation button. Under the User Management heading, find the user you'd like to deactivate and then click on the three vertical dots in-line with their name and click deactivate.
If this user is used in any automations, it will prompt you with the names of the automations they're used in. In this menu you are able to reassign an agent to takeover their role in these automations and then click the Replace button - after which you can Deactivate the user.
After which you can then Delete this user once their automations have been reassigned.