Agreed with Brian. There is generally no reliable way to terminate another thread or process without cooperation from said thread or process. Especially in the case of threads, terminating a thread while leaving the process alive may leave some resources (including Python interpreter resources) in a consistent state. Admittedly, terminating a process could be implemented using kill().

The solution is to write your long-running task so that it can be cancelled using a synchronization object (such as an Event).