You are able to cancel an accepted mission.
Go to the My Missions tab and select the ACTIVE mission you want to cancel. Hit the CANCEL button on the bottom left corner. Confirm you want to cancel and exit page. Mission should now say CANCELLED.
Once the mission is cancelled, another Snooper will be able to accept and complete the mission. If you want to retry the mission and it is still available, open the mission and hit ACCEPT again.