It seems that these kinds of policies are all too common at malls. I suspect that most malls are owned by non-Utah interests.
In addition to contacting the mall, you might also contact the individual owners of the various stores and shops within the mall that you patronize. They are far more likely to be locals who are more accepting of local culture than some large, faceless corporation located who knows where.
In both cases, you are likely to win some points by making clear that you recognize their legal right to limit firearms on their property if they so choose. Then go on to attempt to persuade them that they should not be banning firearms even though they have the legal right to do so.
As a last resort, encouraging more visible signage will at least result in more consistent adherence to and enforcement of their policy. And if they are gong to ban law abiding citizens from carrying guns, they really should beef up their security. It isn't like St. George is immune to gang and other criminal activity.
At the end of the day, I do my best not to patronize locations where my gun is not welcome. If I do patronize, I legally conceal and then their policy never comes up. It has no legal weight in Utah other than giving them a non-discriminatory excuse to ask someone to leave, and failure to abide that request could result in a trespassing charge.
But when possible, take your business elsewhere and if you can find or make the opportunity, let the owners of those stores know why they have your business (not banning your gun) rather than you going to the mall that does ban guns.