I deleted the infoboxes, but when i choose "infobox" in "Add a Page", the infobox still appears in there! I heard a lot of people having the same problem, but the admins keep closing the threads because they're too old.
Based on my brief investigation, I surmise that you are using VE. It is known to be the least reliable of the editors and often takes time to update its menus. I suggest waiting a day or two. If they don't disappear by then, contact Wikia to ask for help or to report this as a bug.