Nope, not as far as I know;
In default.asp, there's this code:
%> <!--#include file="cal_default.asp" --> <%
Removing that will turn it off.
You could write a simple mod for that, and include it in the Forum Features page.
For instance something like this:
IF strShowCalOnDefault = "yes" then
%> <!--#include file="cal_default.asp" --> <%
end if