There are measures that can be taken, and are taken, to stop such things. I am not able to give details, which are not public for obvious reasons, but, e.g., (automatic) checks and restrictions are put on IP address from which a lot of poor content is created.
That is to say, to constantly flag, close, downvote, etc., this stuff is not completely useless.
Yet, we also need to recognize that a sufficiently dedicated person likely will manage to repost and repost and repost for a very long time. Don't get frustrated and just keep removing the poor content. Eventually they'll give up or some mechanism will kick in that stops them or at least slows them down. (Of course, eventually, a new like minded person will show up.)
If you find a clear pattern it can be a good idea to point it out to moderators via an "other" flag on one of the posts.
As mentioned in a comment, what you should not do is flag as a spam (as it is not real advertising anything and only this is considered as spam here). If you want to use a drastic flag use "rude or abusive" (in practice it is very similar to a spam flag). There is some debate if this is an appropriate flag. But I do think it is if there is a clear pattern of reposting the same thing many times. It is rude and abusive behavior even if the content of the post is not.