The Crank arm on my Bike Keeps coming off. How can I fix this?

Okay I started riding my bike again and I had some parts stolen and that one of those parts was the Crank Arm/Rod and I ended up using a replacement from another bike every time I would ride it. The screw would come loose and the Crank Arm/Rod would come off, I tried to put it back on but the Crank Arm/Rod was striped and completely ruined. I decided to take the bike to a repair shop Got a brand new Crank Arm/Rod, The guy put the screw on as tight as he could but the next time I rode the bike the screw came loose and the Crank Arm/Rod started wobbling I checked and the Crank Arm/Rod is a little striped but I can fix that I just need to know how to keep it on the bike I am really close to just gluing the screw in to prevent it from moving but I know that will ruin the bike I am getting to the point of getting a new bike but this is the only problem this bike has every time I took it to a repair shop they just tightened the screw really hard and it never worked. Can anyone help.

put locktite on the bolts

stop useing your arm to crank so hard get a gf