To stop earning Guild Guardians cards in Guild Chests the Guardian must be Ascended to Mythic and you must have 4 copies of it.
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 your Guild Guardian to Mythic and have 4 copies of it, you will stop finding that particular Guild Guardian in the Guild Chests. If you have upgraded your Guild Chest with 20,000 + Seals and are still earning common cards, ensure that the Guild Guardian you received is Ascended to Mythic. You must have an additional x4 Copies in order to stop seeing it in Chests.
There are 12 Guild Guardian Troops and you can find them by using the Troop Filter in the Troop Menu and setting Kingdom to "Guardians".
The 12 Guardians are: