Algodoo

A - Turn on water flow.
Z - Turn off water flow.

type the following commands into the console to remove the key bindings.
Keys.unbind("a",());
Keys.unbind("z",());

Make sure to press z before you pause the simulation, as the script creates 2000 particles a second, and as a result pausing will not have the deletion trigger, so you will need to press z and pause the flow, otherwise the particles will continue to be added, it should be fine to leave it on as long as you erase the water before you begin the simulation, as water isn`t very laggy when paused, but when you start the simulation, the contacts will be calculated and there will most likely be a resulting lag.