To stop earning Guild Guardians cards in Guild Chests all Guardians must be Ascended to Mythic with an additional 4 copies of each.
Guild Keys are also the only source of a set of Troops called Guild Guardians which appear in Level 4+ Guild Chests. Guild Guardians appear to be same as Common Troop cards but have a 50% chance to appear in any level of a Guild Chest. They are special cards with unique abilities that you won't find in common cards, hence they are frequent in Guild Chests.
Why am I still getting Common cards in Guild Chests?
Once you have Ascended all your Guild Guardians to Mythic and have 4 copies of each Guardian, you will stop finding Guardians in the Guild Chests. If you have upgraded your Guild Chest with 20,000 + Seals and are still earning common cards, ensure that all Guild Guardians are Ascended to Mythic. You must have an additional x4 Copies in order to stop seeing them in Chests.