There have been many issues with people's spawners and spawner setups on the server causing lag. So here is a basic guideline/rule set for spawners.
You can use either autospawners or spawners purchased at /warp voteshop. However here is a list of rules for them.
1. Spawners must have some form of off/on switch be it levers or rednet cables. Whatever you choose. But it most be able to be turned off or staff will destroy your spawners with no refunds.
2. Mobs must be killed at a faster rate than they are being spawned. The build up of mobs lags the server. If the mobs are dying within 5 to 10 seconds you should be fine.
3. Mob farms cannot be chunk loaded. You can use iron block loaders, but not diamond block loaders.
4. If your mob farm is causing server lag staff will break your spawners and there will be no refunds.
Most importantly please just use your common sense. If it's laggy find another way to set it up. Grinders seem to be effective. If you need information on grinders check this wiki. http://ftbwiki.org/Grinder_(MineFactory_Reloaded)
If you have any questions feel free to ask.