A hanging note (i.e. a note that continues to ring even though you have taken your hand off the keyboard and are not holding the sustain pedal) is the result of the computer receiving a NOTE ON message without it's subsequent NOTE OFF message.

This is typically due to a malfunctioning MIDI interface.

If this only happens every once and awhile, you can click the MIDI RESET button in RMS Keyboards (found under the Windows menu at the top of the program) or hit Command/CTRL + M.

If this happens constantly you need to investigate your midi signal outside of rehearsals and performances.

To do this, download MIDI Monitor (OSX) or MIDI OX (Windows). These programs track all MIDI entering your computer. After getting set up, you'll notice that every time you strike a key on your physical keyboard, you're given a NOTE ON message in this software. When you pull your finger off the note, you're given a NOTE OFF message. As mentioned earlier, a stuck note is a MIDI ON without a MIDI OFF. If you're seeing this, then you should investigate your MIDI interface - this is the root of the problem. You can try plugging the midi interface into a different USB port and testing again. You can try a different MIDI Interface. If the keyboard supports USB out (or USB to Host) you can pull the MIDI interface out of the equation completely and just use a plain USB cable to connect your keyboard. At that point you may need to download a driver for your particular keyboard as well.