How to terminate a query

Hi there!

select * from pg_stat_activity;
shows me a
select my_function(....)
query that has been running for too long.
How do I kill it?

kill -9 of the procpid seems to kill the entire server process. So
I'm not really comfortable with that.
The query was started by a webscript that was closed a long time ago.

So how do you stop them? (This is on a Linux box)