There is no exploit - as it is not an exploit to never again visit the faction overview screen of a given faction (after having reached a certain faction standing with that given faction). In case you are wondering, how and by which means a faction can be pushed to a standing value above a cap: If there are ways to raise standing (with a faction) in other ways than by donations, then it is usually an uncapped standing increase (there are exceptions). Also not for all factions there are ways to increase standing by other means than donations.