"Hack" the JS file on just your own machine? There are plenty of tools that allow you to modify JS code on any page you visit. One of the more popular ones is Chrome's developer tools (press F12). You could also download all the page resources (including the DTM scripts), and make any modifications to the Analytics implementation yourself via the local copy you just downloaded. You could alter any eVar value that you'd like, but such is the nature of any client-side JS code.
Hack the JS file remotely so that it applies to all machines that visit your site? The only way I see that happening is if your DTM account was compromised, and your DTM account had the permissions to edit and publish changes.
If you're truly, genuinely concerned that everyone that visits your site is going to commit data fraud and start sending nonsense to your report suite, you are welcome to look into a server-side implementation. All links within the Web server row on this page (Developer) are options that you can use. Web Server implementations would not allow the client to see or alter Analytics data. However, the need for DTM at that point is minimal, since DTM is for client-side implementations.