Descaling a Keurig

I have a Keurig at work that badly needed to be descaled. I brought in vinegar and did like it said and it worked great. for 3 days only. Then same problem. So I repeated the process and worked great again for another 3-4 days. We only use distilled water to brew at work. Well anyway the 3rd time it was plugged up and asking me to descale yet again (all in a month) I called the company (3rd party coffee program - I order supplies and machine is free) and they sent a new machine overnight and let me keep the old one.
So now this thing is in my garage and not wanting it to end up in a dump (yet anyway) Is there anything more permanent I can use to descale it? maybe the vinegar isn't strong enough?
I am really tempted to run some of my muriatic acid through this thing. I know it will descale it for sure but might also break it more. Any other ideas?

Some good replies. First, I already have a working Keurig in my own home. I use distilled only and it never plugs up and never has issues. This machine in question was acquired free from my medical facility. I order the distilled water, and I personally use that but I cant guarantee some of my staff isn't just filling it with tap water. That may explain the problems. I did tape a sign on the new one that says DISTILLED WATER ONLY. Hopefully they comply.

I have a French press, and I do love it and use it on occasion. But at 5 am Monday morning its a miracle I am even able to push the button on the Keurig and place the cup under it. lol.

I like the CLR idea, and I will try that first over the muriatic acid. If I wound up using the acid, I would run like 10 gallons of water after to be sure its all gone. plus this particular machine has a "drain" function which purges it of all liquids. so pretty safe on that front sort of having the acid dissolve the whole machine. Def. a project to do on the lawn vs the kitchen counter. love those projects!

thanks for CLR advice. will try this Saturday.

ah yeah I read about trisodium phosphate and actually went to the local bbs to go pick some up. turns out the EPA got to them too and they have some phosphate free trisodium phosphate. I guess i'll have to go to amazon or some website to get it. wanted it for the dishwasher.

No, electrical tape on the light wont help, lights on do not bother me. In the winter I run snow tires on my car but they are on steel rims and I did not get TPMS sensors, so the low tire pressure light is on all winter. Its an amber light, kinda decorative. It reminds me that I have the snows on and to keep the speed down too.

LOL @ "water dingus"...

Tomorrow is the day I hit the machine with CLR. will post results. I got the ZEP version, which I've had better results with. I think CLR also got hit by the EPA because it doesn't work as good as it used to but ZEP does.

I wont wait until its fully stuffed up. Its been in my trunk all week. I'm hoping to fix it so that 1. it doesn't end up in a landfill, 2. as a backup machine in case mine breaks 3. Could use it for company as brewing 1 cup at a time stinks. I guess I could just brew a whole pot in the other machine.

Update - Filled the thing with CLR and ran it through 5 times. then let it soak a whole day and ran it through 2 more times.
Flushed it out with clean water many many times until i could no longer detect the clr smell. Seems to work perfectly. Its now in a closet for use as a backup when the "good" one breaks.