Make a backup of you site. Making a local copy of your site on your computer. Then once the site is up and running go into the database and see what database tables the entries are being written too. Then mass delete the entries within that table. Make sure the site is fully operation once you have deleted all the entries. After that look in the marketplace for a recaptcha add on and add it to the site form. That will help avoid site future span messages.

Most spambots look for generic contact forms to submit to. To help prevent too many spam submissions, you may want to consider using a CAPTCHA.
Also, you might want to consider adding an additional input field, one that a bot might not pick up on.

Use a «honeypot»: In your form/form handler, add a hidden dummy field to your form and name it eg. «VorName» and check if the field was submitted with any data. If yes, it is probably a robot. Visitors filling out the form do not see the field and therefore wouldn't have entered any data to it. Or else, you could add a hidden text field, positioned eg. absolute left -3000pixel.