I'm a little late on this response, but by default the Boost is set to zero, which means it is already turned off unless you went in and turned it on.
Remember that the Boost is actually comprised of both Boost Interval Time and Boost Temperature. Boost Time can be changed, but Boost Temperature is fixed at 9F. The Boost Time simply raises the heating water set point by 9F if the thermostat is not satisfied after running for whatever the Boost Time is set to.
There is nothing about the Boost feature that will "boost the heat capacity" of the unit, at least not max heating capacity. The maximum heat capacity is limited by the maximum firing rate.
It sounds to me like you are looking to limit the maximum firing rate. When the unit fires on a call for heat, the burner will come on and ramp up the firing rate very quickly (up to the maximum firing rate) to try to achieve the heating water set point. When it gets near the set point, the firing rate turns down to maintain water temperature. You can limit the Space Heating Max Capacity - see page 66 of the manual. This will limit the maximum firing rate, causing the heating water supply temperature to rise more slowly.
