lench - if they are your usual script kiddie then they're prolly on windows & prolly still have the msging on so use the netsend command

at a dos window type netsend theirip yourmsghere

you could type something like
ip scaning detected by Microsoft.com we have alerted the authorities

it'll just pop up in a wee box on their screen - hmmm at least i think thats the command!?

v_Ln