The Site Manager has two ways to block spam that may come in through the email form.
1) Blocking IP addresses of spammers. Every time a form is submitted, the IP address of the submitter is logged. You may block IP addresses which have been submitting spam under "Administration" -> "Security" -> "Blocked IP Addresses".
2) Blocking email addresses, domains, and/or keywords appearing in the subject or body of the message being submitted. You can set up these blocks this under "Forms" -> "Spam Blocking". Here you may enter email addresses, domain, and/or keywords that you would like to flag as spam, preventing the form for being submitted.
You may also see a list of all the messages submitted through the email form under "Email" -> "Message Log". This can be useful to see what IP address, emails, domains, and/or keywords need to be blocked.
This should help to reduce recurring spam through the email form, however, if you need further assistance or have any questions, please just let us know.