It's not a limitation of the internet, it's a fundamental property of communication.

Imagine trying to validate that all letters sent to your company are written by special company-provided typewriters and you would run into the same fundamental limits.

Whenever you design any client/server architecture, the first rule should always be "never trust the client," for that very reason.

Rather than trying to work around that rule, put your effort into ensuring that the system is correct and resilient even in the face of malicious clients.