It is NOT ALLOWED. Period. The fact that it is so wide-spread does not make it permissible! It SHOULD cause pain to a sincere Yid praying there. If it interrupts your concentration, double that! I, for one, do understand your anguish and frustration. I daven in a shul that has absolutely NO TALKING – from beginning to end. When in another one, it does grate at my nerves.
Two ideas come to my head (klain-kepig); try what it says (in halacha?) to scream and rebuke them. Maybe come prepared to shul with a printed placard stating the halacha. Keep it by your place, quietly. Second option, if all else fails, try to block out all distractions, know before who YOU stand. Then realize that all the yapping is their problem NOT YOURS.
btw It is permissible to don a hat and jacket in deference and awe for HaShem when davening. It’s called ??? ????? ????? , don’t want to sound scholarly, but that’s what they taught me.