Is there a way I can turn off the D-League? I don't want it as part of my league.

No there is no way to do this

Is there any way to adjust the number of games in the schedule?

There is a schedule.dat file you can open with a database program and edit the schedule for the d-league but I would not edit the schedule outside of the start and end dates - in other words don't put games before the first game date or games after the last one but if you want to eliminate some or add some in the middle it should work ok

Is this the same process for the pro league as well?

Yes but again be careful not to mess with the all star dates either if editing the pro schedule

How do I determine what the all star dates are in the .dat file?