Hello,
If I understand correctly, you don't want the filter prompt to affect the crosstab. Is that right?
A report prompt filter will always affect all of the objects in the report. If you move the filter control object from the report prompt area to the main body of a report page, you can select which objects it interacts with. If you want the filter to affect objects on multiple pages, I think you'll need to assign a parameter to it.
There are probably additional tricks and workarounds to this issue out there.
Sam