Look for TerminateThread() WinAPI function.
Some useful explanation can be found here or look at MSDN documentation.
Of course, after terminating you must look if any resources allocated in thread not freed and free it appropriately.
Update
Yes, using TerminateThread is bad practice (as specified in comments). I'm agree with this opinion. But "never use it, even if you really need to use it" recomendation it too strong from my point of view and very theoretic. Real world full of design flaws and buggy 3rd-party libraries.
Information, given in question not enough for making right decision about this concrete situation. E.g. it may be temporary workaround with no alternatives, etc.
Therefore, from theoretic point of view right answer is : "There are no way to terminate process properly if you can't control how to "freezing" step in background thread processed."
From practical point of view right answer is: "There are no way to terminate process properly if you can't control how to "freezing" step in background thread processed. But if you realize that you can't, but still needs such functionality - use TerminateThread() API call"
About TerminateThread vs. TerminateProcess:
- Creating/terminating process requires more resources than creating/terminating thread
- Creating/terminating process more complicated => more place for bugs
- TerminateProcess don't terminates immediately and waits for I/O operations to complete (MSDN) => not a choice for scenario where remote shared folder becomes unavailable while reading and other similar I/O scenarios.
- Creating and terminating process requires more user privileges than creating thread, compare MSDN here and here
About resource freeing:
Thread stack freed automatically when terminating thread (as mentonied in MSDN). Resources is primarily resources, allocated by main thread for communication with background thread. E.g. memory structures, mutexes, etc.