Though we currently do not offer specific SPAM filters you can control the email aliases you allow your domain to receive email from.
Remove the default line in your .redirect, this will block all email from coming in except aliases that are specifically listed in your .redirect file.
Example: To receive email to your domain only from sales, billing, and support.
Make the following entries. (Assume your domain name is yourdom.com)
support yourdom@yourdom.com billing yourdom@yourdom.com support yourdom@yourdom.com
Now you will only receive email to the listed addresses.