How to remove all list item permissions using PnP Powershell

Arioule 

It's normal behavior for the current user to remain in the list item permissions when the inheritance is broken and cleared on a list item to ensure that at least one user still has access.

Try this:

First assign permissions to a user:

Set-PnPListItemPermission -List 'ListName' -Identity 1 -User 'user@contoso.com' -AddRole 'Contribute' -ClearExisting

Then remove permissions of the user running script:

Set-PnPListItemPermission -List 'ListName' -Identity 1 -User "scriptuser@contoso.com" -RemoveRole "Full Control"

Reference: Set-PnPListItemPermission -ClearExisting adds the user running the script with full control  

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