I'd like to stabilize the gimble while flying in sport mode. Usually when flying in sport mode I don't care about video and I'm just flying. But it sure seems to put a fair amount of stress on the gimble. Is there a guard or stabilizer that I can use to "hold" the gimble in place?  Is it even a good idea?

There is no such product. It's never a good idea to attempt to restrict the gimbal movement while the Spark is powered on.

The gimbal already locks it self in place in sport mode. Thats why when you go left and right the whole video goes sideways and tilts.

Don't do it.   You will get Gimble overload and heat things up..