A system and method to control sending of unsolicited communications over a network is provided. Typically, the network is the Internet and the system defines a SPAM prevention tool for users of an on-line service, such as an on-line bidding service. Users may contact an intended recipient with a contact form, which is forwarded to the recipient's e-mail address by an on-line service provider. A link accessible from a Web site allows recipients of unsolicited e-mail to complain to the on-line service provider. Sanctions are implemented against the sender of unsolicited e-mail based on the frequency of complaints. When the complaint level against a sender reaches a certain threshold, the sender is unable to use the contact form for a predetermined time period.