infinite blocking statements in 8.2.3

I'm running 8.2.3 on ubuntu 6.06 (2.6.15-26-server SMP i686)

sometimes i have SELECTs that never ends. Normally I drop connections by
killing the connection process (kill <PID>). But these hanging
connections (which blocks other statements infinitly) cant be killed.
the only way is a pg_ctl -m immediate stop or a brutal kill -9
what can i do to
- limit statement runtime (set hard timeout)
- remove these blocking connections without killing other connections

Thomas