In an elevated PowerShell session, adjust the following to your environment and execute:
$WSUS_server = 'wsus-srv-01'
$UseSSL = $True
$Port = 8531
[reflection.assembly]::LoadWithPartialName("Microsoft.UpdateServices.Administration") | Out-Null
$Wsus = [Microsoft.UpdateServices.Administration.AdminProxy]::GetUpdateServer($WSUS_server,$UseSSL,$Port)
$client = $wsus.SearchComputerTargets("host-01")
$client.delete()
This removes the server named "host-01" computer object from WSUS on host wsus-srv-01.