[SeanDav]

From the sounds of it - just change your group description to something not descriptive enough and hey, presto, one perma-deleted group.

[PanMan]

We still have >4K members, and still have to keep paying to keep them from getting spammed.

[ajays]

Spam them yourself, telling them to leave the group; tell them what will happen when you stop paying; and if someone still sticks around, they can face the consequences.