Open an xterm and type in

killall browserd

That should kill all running flash sessions.

I do not suggest running

killall -9 browserd

or

killall -kill browserd

on my N900 the -9 switch caused the phone to reboot. There are instances of browserd that run as part of Maemo.

A more precise method is to open a X-terminal then type in

top

the applications using the most cpu will be at the top of the list. Find the PID for the offending program which is in the left column of the list. Then press q to exit top and run

kill PID

If that does not work then run

kill -9 PID