Answer
A system parameter controlling whether Person-record creation happens automatically during provisioning was introduced starting around specific SUs across several release tracks (roughly the 22R2, 23R1, 23R2, and 24R1 SU lines, and the 24R2 EA) — check whether your current release includes it and, if so, set it to suppress automatic Person creation. Note this only stops a Person from being auto-created; supplying an existing Person's ID directly through SCIM provisioning to link a new User to that already-existing Person record (rather than just suppressing creation) has been reported as still not working as of this writing, so don't assume that specific linking scenario is solved just because the create-a-new-Person-or-not parameter exists.