How do you kill a Thread that is only running one command permanently?

Question

I have a thread that is running a runnable with just one command like this:

mainRunnable = new Runnable(){
    public void run(){
        startClient();
     }
};

This is not a normal thread where I am doing something over and over again (this would be easy to stop), but a thread were the command is running constantly and is blocking execution of further code in the runnable. So I basically, cannot implement the stop I believe.

This one command startClient is running all the time, is there a way I could stop it using the Thread object?