ip address reserved

i am sharing a DSL connection among the people in a building through DSl router & switch. people using the internet get there Ip address automatically through the router and the reserve the IP address for themself, so that next time when they log onthe are able to the internet. i had changed my Default gateway ( router IP address) many times , but agian they do the same thing. How can i find out who is reserving the Ip address.