To quote the page you linked describing the license:
The licensor cannot revoke these freedoms as long as you follow the license terms.
Because posting on the Stack Exchange network grants such an irrevocable license to the content (see "Subscriber content" here), a user is unable to forbid redistribution of material they posted on the Stack Exchange network, as long as the person doing the redistribution follows the license (including, for instance, a link back to the post).
You are not required to ask, because the license has already been granted. Even if they ask you not to redistribute it, you still can, because the license cannot be revoked.
One exception: if the user did not have the right to license the material to Stack Exchange in the first place (for example, it was copied from somewhere else), then there was never a valid license in the first place, and the actual copyright holder could enforce their rights. The user should never have posted this material on Stack Exchange in the first place, but occasionally people break the rules.