What browser are you using? Could be a browser setting to NOT save info you have entered and pre-populate it on other websites. Of course, that might stop pre-populating on ALL websites. 'Cookies' may be coming into play as well from the website(s).
How do I block specific unwanted websites from entering my email?
Do you mean what jav6joev is referring to? Or did you mean you want to block incoming emails from a certain sender?