There is a PDF guide which explains the installation instructions for Cadence libraries. You can use it to uninstall the libraries from your computer because there are uninstall procedures explained as well.
Download PDF: ftp://192.220.82.61/FDS/Allegro%2016.5/Disk1/documents/pcbInstall.pdf
Note: Uninstall instructions are provided on page 26 in the PDF manual.