Can you move the subreport out into the main report?
Think of Crystal as processing like a printer. If the subreport says a new page, which I have never done, then the report should give a new page, but check won't say void, because Crystal is still in the subreport. When Crystal exits the subreport, it will continue with the main report.
If you move the subreport into the main report, you might be able to control this behavior to a better extent, but as I think, probably not.
Is this on a preprinted form, or are you generating the check in the report? If this is on preprinted, I can't think of a way to control Crystal to print all of the details and the check be voided. If Crystal is printing the check, it would seem that there should be a way to print your details, make a new page, reprint any header info and continue the details and print the check at the bottom...it would take work, but it all depends on how the check is printed.