How to stop a program from accessing the internet?

I am trying to use Garry's Mod on Steam with the Undead Cracker by SASiO and i have learned that when i start up Gmod it takes me to this error. after doing research on what it meant, it means that it found out that its invalid and it wont let me play. the only way to remedy this is to disconnect from the internet, but i dont want to have to turn it on and off and on and off, so im trying to make the program stop from connecting to the internet. Can someone help i wanna get to playing