What you want isn't reasonable. Giving root to people in any form will allow them to wreck your system. Even if their access is via heavily regulated sudo, a clever or malicious or really deranged idiot will be able to cause mayhem.
You either trust them or you don't.
If you're supporting their needs and the firm as paying you to support them, it is their machine, not yours; you're only there to keep it working despite whatever foolish things they do. Maybe after they roast it a couple times you can ask your boss if you can give the clients less access or otherwise the firm will have to keep paying you to fix the mess. Whatever -- it's their dime.
If it is your computer and you're doing these others a favor, ask them politely to be nice.
That said -- you can always boot from CD (or other read-only media). They'll still be able to wipe out the other filesystems or mount over /etc with their own /etc, but all you need to do to get the system back to some rudimentary level of functionality.
You can also use something like openvz and give each user their own instance. They won't be able to wipe out the whole system and they'll be able to install their own software in their slice. Lots of places use this for web hosting where they give the clients "root" in the VZ slice.