I want to carbonate several sankey kegs, but the keg couplers have the liquid line open, so if I apply CO2 to them, the beer will come out if I don't have a serving line connected, I only have 2 serving lines, but I want to carbonate all 6 sankey kegs at the same time, how can I block that line? or is there another thing I need to buy/get to close that line?
You simply need to stop the flow out of that end of the coupler. Pretty much anything you dream up that will connect with 7/8" 14 thread will work. If you have a local fitting place, ask for a cap nut, or acorn nut with that threading and you will be good to go. A search on eBay for "7/8 14 nut" had a few that fit the bill for under $5.
A note of safety: If you just cap the end of the coupler, be sure to bleed out the pressure before taking off the nut. The CO2 in solution will remain, and you wont have a projectile coming for you when you take off the cap. Also some Teflon tape will ensure a good fit and will stop the nut from seizing to the coupler
If you want something more commercial, and can pony up some money, there are a series of ball valves that are made to do exactly what you are looking for: