For those of you who don't already know, here is how it usually goes.
1) A cheater is discovered, typically by a regular.
2) He or she calls for an admin by saying something like 'cygnus, thealpha, plop, knuckles, raz0: some_name is cheating!'.
3) This text is shown on IRC in channel #littlewhitey.mtarace.
4) Most IRC clients have what is called highlight, which is a feature that typically makes a sound and colours text in another colour if a message with your name is received. Hence, an online admin will see the message about the cheater quite quickly.
5) If the cheater is known, the admin might ban the cheater right away. If not, the admin will go in game, typically covertly, and try to confirm that the person is indeed cheating.
6) If the admin determines that the person is indeed cheating (this might take a few races, if there are instant-respawn maps up), the cheater will be banned.
Now, the admins typically won't reply when you say someone is cheating. This is on purpose! We don't want to scare away the cheater. Just have patience.
Also, please don't votekick. Admins don't always notice these (lack of highlight). It also serves no real purpose, since whoever gets kicked can just join right away again.
Furthermore, the admins are probably more online than you think exactly because many of us like to play covertly. :-)
Now, I agree. There is a lack of admins to monitor the server at night sometimes, and if we were to expand with more admins, I would definitely vote for someone in an American time zone.