Lock/secure VDI

[DfDf]

I wonder is there ANY way to lock/secure VDI from changes (for ex, a password or smth. alike)?
We need to give our users preconfigured VDIs on portable HDDs, but they can move them home, open/mount this VDIs on home Virtualbox installation and change [thus damaging] the image, losing functionality.
So I wonder how can I prevent such changes to VDIs?

Vmware has VMWare ACE product, but it's to slow cause entire image is encrypted (moreover, it's not free )
All I need is to prevent any changes to immutable VDI (including converting it back to normal).

Is there any way to achieve this?

[mpack]

There is no way to prevent someone from modifying the base image, if they are detemined to do it. However it should be possible to create a self contained VM based on one immutable image plus a difference file. The immutable base can then be made read-only and hidden, and it will still work.

This doesn't actually prevent anything, but it should make accidental damage more difficult. Deliberate sabotage is of course another matter.