How to disable battery charging?

Solution 1:

If you use a small piece of tape to block the middle (signal) pin on your MagSafe charger, your Mac will not charge because it can't identify the charger.

Solution 2:

Not an Apple-approved answer, but to solve your root cause (to train battery regularly when your MBP is always at your desk), consider surge protectors with scheduled on/off (example, about $40 in Russia). You can program it to turn off in the beginning of night, and to turn on about 4am so that battery is charged back by the time you show up at your desk. To save charge cycles, I believe it should be performed not daily--but maybe once or twice a week.