Effectively nothing can stop a computer tech from snooping into your data once you hand your computer to them. It's a very real risk and there is littel you can do about it. Your best option is to get someone you know and trust to look after your computer without any second intention.
Or just learn the tricks yourself and get rid of the techs altogether
It's a very real problem and applicable in any field really. A mechanic repairing your car can "spy" anything in it or do a negligent job or not do anything at all and lie to you. Or what prevents people you hired for repairs in your home to rob you? In any case some degree of trust is a must, and I would say that's better if you know the people beforehand or some relative or friend recommends him.
Back to the computer field, a partial solution is to put all your private data in a second physical disk and pull it before shipping for repairs. I think it's a poor solution because you first need to know how to pull the disk yourself, and it may misled the tech if the problem was caused by that second HD. Also, private data many times stays in the main HD anyway. And for owners of many notebooks, two hard disks is outright impossible.
Full disk encryption is another technique used to prevent snooping. Mostly useless here too, if the tech needs to perform some software repairs he will need access and the description key, hence defeating the purpose, or he cannot properly perform his work.
