Hello,
When your devices are no longer linked to Workspace one, you can hard reset you tab or phone on Android. Usually you have to turn off you phone then hold power and volume up or down. Google it to see the specific procedure depending on your manufacturer.
The other way is to use ADB command to factory reset it. You'll have to link your device with an USB cable, but you may have restricted the mechanism in restriction profile.
https://stackoverflow.com/questions/14685721/how-can-i-do-factory-reset-using-adb-in-android
To get ADB (result from Google)
HOW TO: Getting ADB on your PC without installing full Android SDK - Android Community
When you enroll the device again, you can remove it previous enrollment entry and the invitation if it's still recorded, sometimes it helps. I don't know what do you mean by "detected as invalid" do you have an error message ?
Best Regards
Eric