The fastest path is to delete the NimbusMail workspace from Settings. Since you are the Owner, you can delete it now to stop future billing. Your Ghost login should be fine, and if anything breaks we can help restore the link.
Citations
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The customer is Owner and can delete the workspace.
Owner can delete, but the draft omits required SSO decoupling warning.
Ghost login should be fine after deletion.
Policy says deletion destroys SSO linkage materials.
NimbusMail can restore the Ghost link if anything breaks.
Policy prohibits promising restoration after deletion.