Yes there is. You could stop the scenario with Greenfoot.stop(); (Then you can only go on with clicking the run-Button) Or you could change the speed with Greenfoot.setSpeed(...);

Note though that as GameCode said - if you use Greenfoot.stop() you can't resume by a key press since there won't be any code running to check that key press! If you want to go down that route (i,e, resume from a key press), then the easiest way would probably to have a "paused" boolean field in the relevant actors, and invert it whenever a particular key is pressed. You'd then only execute the main body of the act method if paused was set to false.