Suppressing auto-generated .css files with desformat=htmlcss

I've got an 11.1.2 reports server on which I'm trying to generate an html report.  Actually, I'm trying to generate it with desformat=htmlcss.  It generates just fine, and it renders just fine, but each time it generates, the server creates a unique css every time.  For example, if the report I'm generating is "SampleReport.htm", I get a load of css files in the output folder with unique names like the following:

SampleReportHRP0BUV6.css

SampleReportY4WXJ37M.css

After a while, the folder will fill up with these css files.  Our 9i server didn't do this. Rather than creating a separate css file, the style sheet definitions were included inside the generated htm file.