While in the progress of deploying signatures with centralized location in Office 365, I found this piece of code to be used to remove user custom signatures. It will automate removal of signatures in Windows machines. Tested in Windows 2012, 2016, 7, 8, and 10.

RemoveOutlookCustomSignatures.ps1

# Remove signures from users when you dont have centralized server 

$MSLocation =  $env:USERPROFILE.ToString() + "\AppData\Roaming\Microsoft\"
$SignaturesLocation =  $MSLocation + "Signatures"

Try {
    
    "User profile location - " + $SignaturesLocation
    cd $SignaturesLocation
    Get-ChildItem *.txt | Rename-Item -NewName { $_.name -Replace '\.txt$','.txt.bkp' }
    Get-ChildItem *.htm | Rename-Item -NewName { $_.name -Replace '\.htm$','.htm.bkp' }
    Get-ChildItem *.rtf | Rename-Item -NewName { $_.name -Replace '\.rtf$','.rtf.bkp' }

}
Catch [System.Exception] {
    
    Write-Host -ForegroundColor Red "Signature removal failed"
    Write-Host -ForegroundColor Green "This script only supports Windows Vista,Windows 7, Windows 8, Windows 8.1, or Windows 10"
    Write-Host -ForegroundColor Red "Your version of Windows is" ([System.Environment]::OSVersion.Version).ToString()
    Write-Host -ForegroundColor Red "Description of errror : "
    $error[0].ToString() + $error[0].InvocationInfo.PositionMessage

}