Security Hack: How can we prevent members from modifying forms?

[Justice]

Right now, on pretty much any message board, a user can modify forms and insert their own data. This means that the maximum character limits, drop down select menus, and virtually every aspect of your profile forms can be modified relatively easily. It's done by logging into the 'edit profile' page, saving it as an html document, modifying it as you see fit, loading it in your browser from your PC, and clicking Submit from the modified html page on their harddrive. It's simple enough for anyone to do... question is, how do we prevent it?

I need a php command (it'd probably only be a line or two of coding) that would prevent people from submitting forms from outside domains and PCs.

Did any of what I said just make sense?
I just submitted this thread from my harddrive and tried changing the maxlength field from 85 to 120, and the form did submit... but the subject title length was chopped down to 85. This is basically the type of security measure I'd need for profile fields