Deleted junk mails (RCN) keep reappearing

I use Mac OS 13.4 Ventura, with Mail for MacOS. Lately, whenever I go to the Junk Mail folder in my RCN account and delete the mails (confirming by clicking on the "do you want to delete 150 junk mails?" dialog box), they disappear momentarily--then reappear! The only way I can permanently delete them is by logging into RCN via Webmail. (And the reason I use Mail for MacOS is precisely to avoid going through the pain-in-the-butt of logging into webmail, which is an infuriatingly redundant time-waster). Is this something I have to contact RCN to fix? (I hate doing that because their customer service reps are incomprehensible and generally incompetent). Is this something I can remedy in Mail's Settings?