Sure. You can deactivate a discount rule anytime.
However keep in mind that the more target products, variants the rule has, the longer the discount deactivation process will be.
To cancel discounts without waiting for the rule to be deactivated according to the schedule (if such a schedule is configured) you should disable the rule.
How long does a discount rule apply to products and variants?
Will the app return prices after discount rule deactivation?