I don’t think it’s possible to evict cookies of other websites. But we have other options here
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From your Patreon account page
https://www.patreon.com/settings/accountyou can unlink the Discourse app which is wrongly connected. -
While sign-in again using Patreon in
/oauth2/authorizepage where you will be asked for either “Allow” or “Deny” the permissions, click the “Terms of Service” and logout from Patreon in next page. (yes they are missing “logout” / “switch account” link in authorize window)