How to control memory usage of postgresql

Hi,
I have a postgreSQL database running and sometime ago there was a small 
problem, some query running inside the server was onsuming too much 
memory, so the server (a DEBIAN LINUX) became out of memory.
 To solve that, the server must drop all queries that start using a lot 
of memory, before the server get out of memory again, how can I do that?

Thanks in advance,
Andre.