Is there a way to make an iPhone completely useless?

This support article has a list of things to do if your iPhone, iPad, or iPod touch is lost or stolen. --> If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

Make sure you set lost mode and Activation lock.

Yes, login to your user account from any other computer and use Find My to put the phone into lost mode (activation lock). Do NOT erase it. That will allow whoever has your phone to use it as new.

https://www.icloud.com/

Edit: After you do that, you may get a message or email on another device claiming your phone has been found by Apple/Police/whatever. And all you have to do is follow some link that will ask for your Apple ID and password. DO NOT ever follow such links. That message is from whoever stole your phone and are hoping you'll give them the info they need to remove the activation lock.