For people who have quite often many items to delete, you have a gem, written by Snarlygribbly: it's called "Scene Fixer", which provides many tools, and among them a multi-delete tool.
Head over https://cobrablade.net/snarlygribbly/poser.html and select the one that corresponds to your version of Poser: V12 & V13: first part, V11 second part. This is mainly due to the major change in Python version.

One tool that I quite often appreciate: the possibility to create hierarchical groups directly. First select the group under which you'd like to create a new one, then:

Result:

𝒫𝒽𝓎𝓁

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