I would make sure I have a complete backup of my system BEFORE letting a service center work on my Mac. Actually I try to have 2 full backups, one of which is a clone from SuperDuper or Carbon Copy Cloner. A lot of people have taken system in for repair and have it returned with a completely reformatted disk containing none of their data.
If you have good backups, then you could just erase your sensitive data, as Jeffrey Jones2 suggests. Just make sure you have at least 2 copies of your sensitive data as if you just have one copy you are at risk should something happen to that 1 copy.
I agree these are ok (Lower case) answers but do not solve the original question.
This article is the "Perfect" Solution I think
The method is to create an encrypted sparsebundle ( A Disk Image) and this looks and acts like a complete folder
Open it up
and you can move files IN and out etc
Eject the "disk" and it is closed and encrypted
Double Click to open and you must enter password
and the Disk is mounted ready for action