Your code is in LV21, which I don't have installed so can't see exactly what you are doing, you could try saving for a previous version if you want (I'm on 20 but saving for 18 or 17 will allow everyone running any prior version back to the version you save as to see your code with some limitation).
So, without seeing your code, I'm gonna go ahead and guess that you might be using a loop. If your acquisition is in a while loop it will continuously acquire data until you stop the loop. If you just want one acquisition remove the while loop. It will run once, display the acquired data in your graph and end. There are also likely ways to leave the loop and set the scope for a single sweep on a trigger event like most bench top instruments. It wait in the loop, see the trigger event, acquire one sweep, then stop.
If you go up to the help> Find Examples... type in NISCOPE. There are a lot of examples on how to different things that you can modify to suit your needs. I would look at the niScope EX Getting Started.vi, it does what I think you want, runs and acquires once then stops. theres no trigger, it just grabs whatever is on the scope though.
