My baby is 30 years old so I do not have any need to visit the store. Prohibited carries no more weight than, not allowed or any other “no weapons term” they use. As said so many times on this forum one would not break any laws if they enter past such sign on private property unless refusing to leave when asked. In that case, it is a trespassing issue only.
It is their right to post such sign and your right not spend any money in their store. I believe with such strong wording the only one that may get educated will be the person that open carries into their store. Just my opinion, this might be one where it would be more effective to send them an e-mail or letter asking them why they are not following the state gun laws, when following the state gun laws is the norm for the majority of business.