Remove Office 365 Licenses using Powershell and Graph API. Associated blog post can be found here https://blog.darrenjrobinson.com/adding-removing-user-office365-licences-using-powershell-and-the-azure-ad-graph-restapi/

RemoveLicenseFromUsers.ps1

# REMOVE License Body for RestAPI
 $body = @{
    addLicenses = @()
    removeLicenses= @($LicenceToAdd)
}

# Convert it to JSON
$postbody = $body | ConvertTo-Json

# Remove License from Licenced Users
foreach ($usertolicense in $licensedUsers){
    $AssignLicense = Invoke-RestMethod -Method Post -Headers @{
        Authorization   = $authenticationResult.CreateAuthorizationHeader()
        'Content-Type'  = "application/json"
    } -body $postbody -Uri ("https://graph.windows.net/myorganization/users/$usertolicense`/assignLicense?api-version=1.6" -f $authenticationResult.TenantId)
}