I already removed the device and the people I gave it to still get an activation lock message when trying to set it up.

If you have given away or sold your iPad to a new owner, you need to follow the appropriate procedures documented within this support page:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

This passage may be most appropriate in the described circumstances:

If you no longer have your iPhone or iPad

If the steps above weren’t completed and you no longer have your device, follow these steps:

1. Ask the new owner to erase all content and settings using the above steps.
2. If you're using iCloud and Find My [device] on the device, sign in to iCloud.com/find or the Find My app on another device, select the device and click Erase. After the device has been erased, click Remove from Account.
3. If you can’t follow either of the above steps, change your Apple ID password. This won’t remove personal information that’s stored on your old device, but it will prevent the new owner from deleting your information from iCloud.
4. If you’re switching to a non-Apple phone, deregister iMessage. 
5. If you're using Apple Pay, you can remove your credit or debit cards on iCloud.com. Choose Account Settings to see which devices are using Apple Pay, then click the device. Next to Apple Pay, click Remove.
6. Remove your old device from your list of trusted devices.
7. If you purchased an AppleCare plan for your device, cancel your AppleCare plan.