Sounds like a use case for a 'While' loop in an Advanced Action.
Create two object states for your flashing object to represent Flash_on and Flash_off.
Create a User Variable - say v_isFlashing - and give it a default value of 1.
Create an Advanced Action, check 'Conditional Tab', select 'While' from the 'Perform actions:' drop-down list, and set the condition of v_isFlashing == 1.
Under 'Actions', have your flashing object repeatedly change states between Flash_on and Flash_off, with 'Delay Next Actions By' command in between.
Run this action on Slide Enter.
Have your button assign the value 0 to v_isFlashing.
The script will be looped and executed repeatedly about every second and keep the object flashing as long as v_isFlashing has the value 1. It will stop flashing when the value 0 gets assigned, which happens on click of the button.
Play with the ' Delay Next Actions By' values to make it loop seamlessly.