Might be best to report this to Yourhead Software as it’s a Stacks issue, not a RapidWeaver issue. From what I understood, it’s a bit hard to remove externals because they just repopulate when you use or re-install the third-party stack or theme that you purchased because the externals are stored in those files, but I’m not certain.
Anyway it’s a known issue that Yourhead Software would need to fix as there’s nothing we can do on our end to fix it. There’s a guide below on how to contact Yourhead Software: