The internal regulator is not really suitable for this duty. Probably the charge will fall quickly enough not to bother the alternator but you may prefer a lower charge for the battery.
If you can drop the speed that would fool it, depends on how you are driving it whether you can do this. Another way is to get into the field circuit and add some resistance in the circuit to the rotor. You need to go in series with one of the brushes.
This will reduce the maximum charge and still let the regulator take charge when you reach regulating volts.
You may just find it easier to remove the regulator and excite the field via a series resistor or some lamps.
There have been a lot of posts about using these alternators without the regulator if you google the site.
Flux