Short answer: Set the InteractiveHeight = 0
According to Microsoft:
Soft page breaks are calculated at run
time by the control. Although it is
not recommended, you can disable soft
page breaks by setting
InteractiveHeight to 0.
InteractiveHeight and InteractiveWidth
are used by the HTML rendering
extension to provide the equivalent of
PageHeight and PageWidth. Because the
HTML output format dynamically resizes
a report to accommodate drilldown,
drillthrough, and show/hide features,
the report server uses different
properties to support pagination on
dynamic pages.
EDIT Comment Answer:
In the Layout mode, bring up Properties panel. At the top of the Properties panel is a drop down to select which control's properties you are looking at, select Report You will then find “Interactive Size” that includes Interactive Height property.