Stolen Ipad

Did you not have Find My enabled on your iPad and use a Passcode? If you didn't, you can't prevent someone from using the iPad after it's been taken. If you did use Find My and use a Passcode, no one can bypass Activation Lock to use the iPad at all.

See this link for information about what to do if your device is stolen --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Assuming you had Find My enabled on it, just be sure to leave that device listed under your devices in Find My at www.icloud.com. As long as it is listed there (it will be offline if erased, but still listed as one of your devices tied to your iCloud account) then activation lock remains in affect.