A computer system configured to facilitate a coordinated source blocking method, the computer system comprising: one or more connections to a plurality of computer networks; and one or more processors (104, 106) communicatively coupled to each other wherein the one or more processors, 106) are collectively configured to: receive information from a first gateway, the information pertaining to a network data transmission from a source address, the source address on a first network and the network data transmission intended for a destination address on a second network, the first and second networks not directly connected to each other; determine a score based on the received information; determine a participation status, the participation status indicating the first gateway's participation in a coordinated source blocking method; prepare a first response message for transmission to the first gateway, the first response message comprising an indication of the score, a blocking request indicator, and an amount of time the first gateway is requested to continue receiving from the source address that would otherwise have been blocked; and initiating transmission of the first response message to the first gateway. one or more connections to a plurality of computer networks; and one or more processors (104, 106) communicatively coupled to each other wherein the one or more processors, 106) are collectively configured to: receive information from a first gateway, the information pertaining to a network data transmission from a source address, the source address on a first network and the network data transmission intended for a destination address on a second network, the first and second networks not directly connected to each other; determine a score based on the received information; determine a participation status, the participation status indicating the first gateway's participation in a coordinated source blocking method; prepare a first response message for transmission to the first gateway, the first response message comprising an indication of the score, a blocking request indicator, and an amount of time the first gateway is requested to continue receiving from the source address that would otherwise have been blocked; and initiating transmission of the first response message to the first gateway. receive information from a first gateway, the information pertaining to a network data transmission from a source address, the source address on a first network and the network data transmission intended for a destination address on a second network, the first and second networks not directly connected to each other; determine a score based on the received information; determine a participation status, the participation status indicating the first gateway's participation in a coordinated source blocking method; prepare a first response message for transmission to the first gateway, the first response message comprising an indication of the score, a blocking request indicator, and an amount of time the first gateway is requested to continue receiving from the source address that would otherwise have been blocked; and initiating transmission of the first response message to the first gateway.