Is it "play attacking"?
Sometimes Rizzo gets a little too rambunctious with Fatso, and Fatso usually swats him pretty hard (no use of his claws though, thank goodness) and Rizzo will get the point. Maybe set up playdates for Bella with another small dog so she can burn herself out on an equal-energy player, rather than on a lazy kitty
?
If she is aggression attacking (serious biting) you may need to consult with a trainer to see what they recommend. As far as I can tell, saying no and putting her in a time-out is about the right idea since cat/dog language is so different.