From my knowledge, if there aren't any data printed in the subreport, Jasper eliminates the blank space. In other words, if your subreport would normally occupy half a page, but there is no data, then the empty space is eliminated. I'm not sure this is what you mean by including or excluding the report.
Another solution would be to place the subreport in the main report, and from right-click ->properties -> Subreport tab, you select the "Subreport condition" tab and write your condition there. If the condition is true, the content of the subreport will be printed, otherwise it won't be printed.