as51430.net spammed me, so here is the research for tracking and banning. as51430.net is out of Luxembourg. I did not get spammed by its three sister host names, lux-net-ip.as51430.net, nld-net-ip.as51430.net, and swe-net-ip.as51430.net.
lu-customer-ip.as51430.net found the following IPs:
Further research found the following host names that change often: lux-net-ip.as51430.net, nld-net-ip.as51430.net, and swe-net-ip.as51430.net. Maybe they stand for Luxembourg, Netherlands, Sweden? Here is the complete list by ip address, so you can ban all three.