DROP TABLESPACE fails

I'm doing some tests with version 8.1.4 on Windows XP and am having
trouble recovering some disk space.

The tests involve loading some largeish CSV files in via COPY.  One
of the COPY commands failed and I'm trying to recover the disk space
without having to completely reinitialize things.

DROP TABLESPACE x returns 'tablespace "x" is not empty', but if I
examine the 'referenced by' tab in pgAdmin nothing is listed.  The
tablespace directory contains several very large files.

Can these files be deleted safely with the server shut down?