The problem really here is trust and where do you stop?
If you want to stop admins running scripts, what about if they decide to manually delete all datastores on the SAN, or login directly to ESXi, or use another API to automate what they want to do.
An admin could also create a new account and run everything under that account with full admin rights so just stopping automatic scripts for an admin really doesn't help here.
There is nothing that I know of that stops just powershell running and even if there was, you have all the other examples above to deal with.
The solution is to not give admin access to those who are not trusted / trained.
I now this doesnt give you the answer you are looking for but it does sounds like more a trust issue than something that vSphere software can help with.