Let us say, theoretically, I had a form that posted some data to a PHP page unencrypted. Take this poorly whipped up and incomplete HTML for example.
Code:
<form action=anything.php method=POST>
<input name=somedata value=Lulz> <br />
<input type=submit><br />
</form>
Now, with any header viewer (In my case, a simple FF extension Live HTTP Headers) you can get the data as it's being posted, then modify it if needed and resend the data. I need to know if it is possible to intercept the POST data that would be posted, and then stop the form from loading.

I'm hoping to do exclusively with the client (As I don't have access to the server )

By the way, I am not using this for any online game or anything like that. I find gaming a waste of time (No offense to any gamers) and I would prefer to write the games, not play them

Well, thanks in advanced for any help.

~Kenton