If the installation went far enough to show up in the list returned by ipkg status, you can just uninstall it with ipkg remove packagename. If not, you could try forcing it to install and then remove it:
# ipkg --force-depends packagename
# ipkg remove packagename
If those don\'t work, I would then manually remove each file, as you suggest. Since an IPK is just a gzipped tar archive, you can find out where it put the files by looking at the data.tar.gz file within the IPK:
# mkdir tmp
# cd tmp
# tar zxvf /path/to/packagename
# tar ztf data.tar.gz
./path/to/some/file
./path/to/another/file
.
.
./path/to/last/file
# cd ..
# rm -f tmp
Don\'t forget that if you were installing to you SD card then /mnt/card should be prefixed before each file path given within data.tar.gz