How to disable editing non-latest revisions?

You have to be a product manager and above to do this.

You must be a "Product Manager" or above to perform this action.  Navigate to the container, select UTILITIES --> PREFERENCE MANAGER 

There are two issues at play here, revisions and iterations. If you want to dis-allow checkout of older revisions, this is done with lifecycle states and access control. The older revisions should have their latest iteration in a state that prevents check out. For example, if revision A is released, you access rights should have prevented that user from modifying it (being able to check it out and back in). Now admins certainly have this right but normal members should not. Make sure your users are not all running around as admin and older revisions are being set to an appropriate lifecycle state.

Next, if you are talking about checking out older iterations, that is covered by a preference and ONLY applies to CAD Documents:

https://www.ptc.com/en/support/article/CS22588?source=search

This can easily be disabled. What is allows users to do is open up older iteration (if latest is A.5, go back to A.3), check it out and make it the latest on check in. I call this leap-frogging.

SET the non latest to a state like "Released" where edit permission is not granted 

Hello @Aaronm87,

It looks like you have some responses from some community members. If any of these replies helped you solve your question please mark the appropriate reply as the Accepted Solution. 
Of course, if you have more to share on your issue, please let the Community know so other community members can continue to help you.

Thanks,
Vivek N.
Community Moderation Team.