You probably need to do something like... If UFO has line of sight to player set UFO to iseeyou. Then do a sub event. If UFO is iseeyou then do whatever you want with the beam and if UFO doesn't have line of site to player set iseeyou off. If the UFO is destroyed you can either just completely destroy that object so it's not on screen anymore. That way the ones shot down won't do anything because they are not there anymore.
If you want to have the ones shot down on screen still you might have to give them an instant variable. Maybe something called destroyed. So in your other event when you set it to iseeyou also have it check to see if UFO is set to destroyed and if it's not then shoot beam and obviously it's set to is destroyed ot shouldn't shoot beam. Hope this helps you work this problem out.