If you hold down the options-key while clicking one of the keywords in the HUD, all other keywords will be cleared and only the one you are clicking will be set - I learned that from one of Kirby Krieger's posts.
Question: does it also clear non-keyword selections? How do you clear those?
You are welcome, Raphael. For non-keyword rules uncheck the checkmark to the left, or delete the rule completely by pressing the "-" sign to the right of the rule. You can add a it back by pressing the "+" that will appear.