I've been working indepth with Poser for a few years, and still get nervous when it comes to deleting something. In the end, the safest method I've found is to actually install the item to a temporary folder to see just which files are involved. Then I manually delete the corresponding files from my Poser installation. Even then I've made a few mistakes, but I do have my original files backed up to CD/DVD. And I have a catalogue program which can help me find any needed original file.