Was it a pre-organised race/series that had been sanctioned by event organisers at that forum (or any other forum)? If so, then you should report it to the mod/admin who set up the race and let them take the necessary action.
If not, though, then contacting a moderator at that forum is a bit pointless, because it is not up to them to enforce the online behaviour of their members in anything other than "Official events" any more than it is a GTP moderators job to tell a GTP member how to behave online. All you can do is to let the individual know that their behaviour is annoying you, and hope that they don't tell you to shove it....
We have to remember that the use of a forum membership tag doesn't imply that the forum has any control or responsibility over those individuals. As such, the good name of GT5_Net should not be spoiled by the actions of one individual, who, for all we know about it, might not even be a member there at all...
Zetec-ss
i hope all the other gt5net races aren't like this...this is not a good example this guy is setting his forum
I can also say that I've had as many good, clean races with GT5_Net drivers as I have had with GTP_ racers. For the record, I have started paying more attention to who is hosting so that I can spot any repeat offenders of host-quitting, which, fortunately, I haven't had the misfortune to encounter yet. If the problem persists, though, I'd recommend leaving any races which you notice the person is hosting, and join another race.
edit: I see the convo has moved to an existing thread on host-quitting, so I'll close this... 👍
https://www.gtplanet.net/forum/showthread.php?t=104220