Entering the wrong CAPTCHA code will not disable the submit button as the code to process the response is handled server side not client side.
If the CAPTCHA code was handled client side to disable the submit button then spammer and hackers can still submit using automated bots directly to your process page and bypass your form altogether.
You need to follow step 7 onward in
Integration of Web Wiz CAPTCHA Documentation to place the code in to your formprocess.asp so that the submitted CAPTCHA code can be processed on the server and if incorrectly entered prevent the code in your formprocess.asp being run.