Well, Ernesto. I believe this is not possible with OOB scripts. If you check, there would be script includes for/on SLA table that keep on calculating difference of current time and Start time and based on this calculations: Elapsed time & Elapsed Percentage fields are populated. Now updating these script includes would have a system-wide impact and is not advisable. So, I would recommend writing a BR, that whenever 'End Time' on Incident is updated, SLA for Incident stops and the elapsed time is difference of 'start time' and 'end time'.