Don't delete your additional retainers. They can be left sitting dormant while you are unsubscribed, with all their inventory intact. Use them for long-term storage if you like.

In any case, deleting them doesn't stop the game from seeing that you are eligible to hire up to four retainers.

If there isn't an option to select "no extra retainers", is there a blank slot at the top of the drop-down menu?

Worst case, if you can't get it to turn off AND you pay for your sub in a way that you can let it expire, that will cancel extra retainers and they don't automatically come back when you subscribe again. (In my case, I pay with a debit card and the payment fails if I haven't put enough money on the card at the time when it renews.)