Android - Cut power to a phone without a removable battery?

HTC's solution to this issue is to "Press and hold the Power button for at least 10 seconds, and then release it. HTC One X will be shutdown and then boot again."

This is the only solution I would be able to see. If you require something more, then as a phone modder I would recommend you take apart the phone and manually disconnect the battery. This is rather risky unless you are confident in what you are doing. You may be able to refer to this page: http://htcpedia.com/news/HTC-One-X-How-To-Take-It-Apart.html for a tutorial on how to disassemble the unibody of the HTC one x.

The last safe option you have is to simply wait a day or so (since it is stuck on) until the battery drains completely. From there of course charge it up again and continue with whatever software modding you were doing.

It greatly depends on what conditions made your phone to freeze, but generally a long press (10 sec) on the power button might get your phone to reset itself. YMMV, though.

Holding down (volume up+ volume down +power button) at the same time worked for my HTC desire 826 dual sim mobile(Android 5.0.2). not worked holding down power button for more than 10 seconds or even more.