Your global rebel sentiment is based on liberty bells per turn divided by your total population so you can increase it either by producing more bells or reducing your population. Assuming you don't want to kill off your valuable colonists, the answer is to employ more statesmen, especially elder statesmen, and build newspapers.
If you have a large population and few colonies you might not have space for any more statesmen. In this case build some small 'legislative' colonies with three statesmen in the town hall and a single farmer to feed them, and build a printing press there.