If someone is using your web forms to send spam through your site, the problem is with outdated code in your web form. Current e-mail forms do not allow that.
Never expose your e-mail address in any visible HTML code. If you are using an old form, the e-mail address may be exposed.
The problem may not be with your site. There is nothing to stop anyone from using your e-mail address as a return address. Anyone can do that and they do not need to go through your sites.
You need to offer more details in order to identify the problem. How do you know that the spammers are using your sites to send the e-mail?
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin