How do MPPT charge controllers curtail the power if the load is less than the solar generation?
MPPT controllers are not about power balancing, but power maximization. 

Let's say you have a system like this:


Solar panels &rArr; MPPT controller &rArr; battery charger &rArr; battery &rArr; load


If the load on the system is smaller than the incoming solar power, the battery voltage will rise. 

When the battery voltage reaches a point where the battery charger decides that the battery is full, the charger will cut out, and stop drawing power from the MPPT controller. In other words, the battery charger will present a high impedance to the MPPT controller.

The MPPT could react to this by presenting a high impedance to the solar panels, effectively stopping any significant power draw from the solar panels.

When the battery is not being charged, the load will cause the battery voltage to fall. Then the charger will start drawing power from the MPPT again, and the MPPT will start behaving like an MPPT again. And the cycle repeats. The system can cycle back and forth between these two states several times a minute, or even thousands of times a second, all depending on the time constants (battery size or capacitance) and hysteresis in the system.

You may have a commercial product called an "MPPT" that has more than an MPPT controller. For example it could have an integrated battery charger and maybe even an integrated voltage regulator for the battery output. Don't let any oversimplified marketing jargon confuse you. The MPPT part of the device only handles MPPT, and if the device also actually charges a battery in a proper manner, then it must also have a battery charger built into it.