If a user wishes to be removed from the system altogether, if your administrator account has this permission, you can use the below steps to do this:
Find the user, this can be either within a list of users, or as a search, as below:
You can then view the details by clicking the result for the relevant user:
You can then use the 'Delete' tab to remove them. You will then be prompted to enter the reason for deletion (such as if this is a duplicate account, or if the user has requested removal).
Once this is confirmed, the user is then removed from the system and will no longer be sent any Alert messages.