Oh I am certainly not disagreeing with the idea that you could (and maybe should) be unhappy with them "tinkering". They CAN do stupid things. Depending on how your network is set up, and your security is set up, the chances of them doing something truly dangerous to your network is reasonably low. Hopefully none of them have full administrative access. If they do....stop them now.
Except for them deleting data files they should not - which is certainly not a trivial thing - if they do not have system admin access you should be OK. Your question seemed to be concerning, hmmm, structural damage, system damage. If they are clever they can do stuff, but it is likely to screw up just their local machine. A reasonable thing would be:
1. have them write a business case for actions they are taking;
2. have them do ALL development, testing (hahahahahaha....fat chance) on machines preferably off the network;
3. submit files with actioning code for review by someone who DOES know what they are doing.
4. tell them - and enforce it - that if they screw up their machines, THEY are responsible. Go ahead and play, but if you do, you are big kids now and any consequences are yours.
Good luck!
Gerry