Very Simple Question

[SnipeTHPS]

Okay I was wondering how I can stop people from taking the source of a page on my site then modify the form and change it then they can submit it from there site

 

Ex. I have a signup in PHP. They can take the source and change it then make it submit to http://example.com/register.php

 

I want to make it so only forms can be submitted from pages on my site.

 

Thanks I need this fast, people are messing with pages!