Firstly, on your registration server WX go to the communities tab under device setup and remove the devices that you dont want in that default community. When they have been removed and you have saved the config go back to the CMS server and do a refresh on the community you imported. If the devices are removed from the registration servers WX/C then if should be removed from the CMS imported community.
If you want to remove a device out of the primary registration server use this command in the CLI.
MGeo, you will also get a complaint in the CMS log indicating that you have more units than the CMS has been licensed for.
Pisa03, when you login to the CMS WebUI, click on Management->Devices->Select community and submit. You will see a small link 'refresh' in the top of the right frame. Clicking it will instruct the CMS to contact the registration server to get the updated device list.