Systems Management

I can't say there is an easy solution. You can block Windows Update via group policy but that will not prevent a user that gets their hands on the MSI from Microsoft. In order to block the MSI you would have to look at restricting users from installing software by using other security restrictions. I'm not sure what your exact situation is but it could take some work to prevent the installation of the MSI depending on how your infrastructure is configured.

Please correct me if im wrong, but i believe you can run multiple version of .NET
Developers then code there application to use the correct version of .NET on the client.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/cpguide/html/cpconTargetingNETFrameworkVersion.asp

So double check with a test, and talk to your developers about targeting the old version of .Net

I agree with kkaminsk to really block its installl 100% you will need to use software restriction policy (group policy).

I was trying to google up the paper I found many months ago on running multiple frameworks but I cannot find it. Here is a link for configuring the Side by Side execution.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnnetdep/html/sidexsidenet.asp