I'm going to say a few things (fine they've been said before), they are our options:
Community Wiki
The answer should, the question is not necessarily a "community" question, as it was genuinely asked by an author. If the author agrees to a CW post, then so be it. The answer shall be made Community wiki. This way, the rep threshold to add make edit is significantly reduced.
Lock the post
The question should also be locked: If locked, it means that no new answers will be able to provided, thus enforcing the use of a single answer. This would likely be the notice:
This question's answers are a collaborative effort: if you see something that can be improved, just edit the answer to improve it! No additional answers can be added here
An example is here.
Don't vote to close, get a mod to also place a historical lock like this:
This way, you can keep the high traffic posts with high votes, but make sure that other users get a notice that this is not an exemplary question, and don't use it. The only issue with this is that you can't edit afterwards.
An example is here.
This question exists because it has historical significance, but it is not considered a good, on-topic question for this site, so please do not use it as evidence that you can ask similar questions here. This question and its answers are frozen and cannot be changed. More info: help center.
Protect the question
This will help stop spam from coming in, but not necessarily a bunch of new answers. These questions require 10 rep to answer, and you don't need mods to do this.
This question is protected to prevent "thanks!", "me too!", or spam answers by new users. To answer it, you must have earned at least 10 reputation on this site.
An example is here
What I recommend:
Lock the post: Doing so makes it community wiki, and ensures no answers can be added.