you mean this type of
beer gun right? Yes it's a bit annoying I would agree. You can put a ball valve into the beerline but this to some extend defeats the purpose of the device. The reason why this type of beer gun foams much less is because the tube is kept full of beer right down to the tip and thus this design requires a seal at the tip. With that said we should probably design a better one ourselves.
I should say we have a device that will automatically purge and fill bottles that will come out at the end of the year so if you wanted to wait a bit I think this new device is probably a good option for you.