Easily preventable on two levels:

1) collocate your own equipment in a remote data center. The customer service staff will simply have no access to it besides being able to press buttons on the box.

2) use non-commodity hardware like Oracle SPARC or IBM POWER or HP Integrity/Itanium.  Then even if they manage to steal it they most likely will not be able to get the data off of it without specialized assistance.

Edit: Also, don't run Linux on those machines, but their native OS: Solaris, AIX, HP/UX respectively.