Guys, it isn't just lag... from what the OP(first post) said, I think he means that he is being pushed out of the map, this makes the server think you hacked(like the lag @
Jowe meant)as minecraft.net can't go outside the map, to avoid this try staying 1 block away from the map's limits and if you can't... well that's the risk you have to take, I'm afraid... If you are getting wizard kicked from respawning then change the respawn key through "esc-controls-respawn-what ever button that you will never press" please remember though that when you play beta these controls will reset so you will need to do it over again.(Not that big of a hassle) If you are having lag wizard kicks then I suggest you try getting a better internet connection,, or try to do as mach said and put your browser as high priority.. if that doesn't work I remember someone saying that wom has less lag so if you are not currently using it I suggest you do that...
PLEASE REMEMBER TO USE /CLIENT HACKS OFF THEN RECONNECT WHEN YOU JOIN THE SERVER THOUGH! I feel this should'v been in a pm and not it's own thread but oh well... please use this for future reference.