Electronic inline liquid flow control?
Look for electrical, normally closed solenoid valves for water. The common and inexpensive ones are typically operated by 12 Volt DC. Several show up on an eBay search, at prices in the $6 range:



This particular one has 0.5 inch threaded fittings.

From the BeagleBone Black, you would need a relay to switch the valve. Look for sugar-cube relays with coil voltage 3 Volts to match the BBB's operating voltage, and drive the coil activation via a MOSFET or BJT from one of the GPIO pins. Here is an example of such a relay.