Once you are done with your load tests, click Terminate Test on the right-hand menu and then click OK on the confirmation pop-up, in order to free the allocated servers and confirm the termination.
From your Load Testing Home page you can verify that the load test has terminated and is no longer active.
Another option is to first terminate all of the load servers, but keep the GUI console open. In this case the load is still terminated, but the load results are available for viewing and uploading to the website. To terminate only the load servers, click on the Terminate All But Console button on the right-hand menu: 
When you no longer need to view the JMeter GUI console, click on Terminate Test button in order to terminate the remaining server.