I haven't found a way to delete the synths you already have but I have found a way to stop receiving them. All you need to do is have your Consumables Inventory full while you're farming keys. If you receive any keys they will be sent to the postmaster. I suggest clearing out your postmaster before farming due to the amount of engrams you get from opening chests. You wouldn't want to get a key and have it get deleted due to the postmaster being full.