To charge the battery you need the current, not voltage (if you are not about lead-acid ones, see comment by @Andy-Aka below). Therefore you should be mainly interested in using current stabilizer.
Voltage regulator could be used as current stabilizer anyway (look in the PDF, you simply need to connect proper resistor as a fake load and insert the chip in series with the battery). Meanwhile you do not need to use 9V regulator for this. The regulator will consume all unwanted energy (as it usually does in voltage-controlling schematic also).
To prevent the back current in this solution you will simply add a diode (perhaps some Schottky's) in series.
If you will use L7809 as a voltage regulator, you could not prevent some back current, I'm afraid. Look at the internal schematics in PDF for details. You will lose the current through internal output resistors etc.
P.S. it will be hard to charge 6V battery if the cell gives less than 6V + regulator_dropout. Perhaps you should disassemble the battery into 3V or 1.5V cells if possible.