Locking down desktops to prohibit changes to the environment

Many thanks in advance

There is no "magic bullet" super-duper lockdown. There are incremental steps you can do to perform this magic. My first suggestion would be to check out Microsoft's very own Group Policy Scenarios lab kit. The idea is that you can check out what Microsoft suggests as some approaches to help get you closer toward a fully locked down desktop. You'll find the Group Policy Scenarios document and exercises here.