How can I prevent the stolen iPad Pro from being used by someone else?

applekeyb0ardsamurai wrote:

• Is there any way Apple can flag the serial number so that if it’s ever brought in for service, it can be identified as stolen?

No. Apple has no stolen device flagging nor does it maintain a database of such things.

• Can Apple still help even though Location Services were disabled?

No. Without location services and find my, there's simply no way to locate it.

• Are there any other steps I should take to prevent the iPad from being used by someone else?

Unfortunately, again, without Location services and Find My being turned on, there's nothing else that can be done.

Tracking it, locking it and erasing it remotely all require Locations services be turned on and Find My be activated.

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPad feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPad has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPad have been lost. Apple will have no further involvement.

If the iPad was a WiFi+Cellular model with an active Cellular plan, you should report the loss to your Cellular Service Provider - who, if you supply the IMEI, can blacklist the device preventing use with Cellular networks.