In order to lock an OS X device a 6 digit PIN must be provided in the payload.
<?xml version=""1.0"" encoding=""UTF-8""?>
<!DOCTYPE plist PUBLIC ""-//Apple//DTD PLIST 1.0//EN"" ""http://www.apple.com/DTDs/PropertyList-1.0.dtd"">
<plist version=""1.0"">
<dict>
<key>Command</key>
<dict>
<key>RequestType</key>
<string>EraseDevice</string>
<key>PIN</key>
<string>123456</string>
</dict>
<key>CommandUUID</key>
<string>2349d04b-d0ba-404b-afae-4863f85cbba6</string>
</dict>
</plist>