There is no easy way to do this in Squeak that springs to mind.

If you're determined, you can do it within the Squeak image. Write some 
code to track the events as they come in, and if too many equivalent 
keystrokes come in too fast, you can "swallow" them.

If your going to do it in Morphic only, hook your event swallowing code 
into the method TextMorph>>keyStroke:. If you want a more general 
solution, but harder to do, look at ParagraphEditor>>readKeyboard. Be 
forewarned -- save your image before you tinker with these methods, 
because mistakes could be quite hard to undo.