To get an output of current settings:
Get-ADUser -Filter * -Properties LogonWorkstations | where {$_.LogonWorkstations -ne $null} | Select-Object name,LogonWorkstations C:\temp\RestrictedUsers.csv
To set all LogonWorkstation values to empty or null run the following command
Get-ADUser -Filter * -Properties LogonWorkstations | where {$_.LogonWorkstations -ne $null} | Set-ADUser -LogonWorkstations $null