You unlink from Ubisofts side.

Go login on Ubisofts site and unlink your Steam account there.

引用自 jrrahmo：

how do i unlink my ubisoft account from my steam account without making a new account and buying games again??

only way is via your ubisoft account. Has nothing on steams end.

Note for new games Steam uses the Ubisoft API in order to call the Uplay software, at which point the Uplay back end will check if the steam account you are coming from owns the game in question

This cannot be 'unlinked' from Steam's end. Since Ubisoft is the one that is check if "steam account A" owns a particular game.

Recover your Ubisoft account