Remove "save in password manager" option in gpg pinentry

I have some libreoffice documents stored with "encrypt with gpg key" option. Today I was quite surprised when the document opened without requesting a passphrase. Eventually I found an entry for my gpg key in the system key and password manager, Seahorse, where the passphrase could be displayed. I conclude that I must have accidentally checked the "save in password manager" box in the pinentry dialog when I logged in the last time. A small mistake exposed a confidential document and my most important password. This appears to be the same problem as here: but in my case free access to the document and the Seahorse entry survive beyond the timeout defined in gpg-agent.conf, after echo RELOADAGENT | gpg-connect-agent and even after a reboot. I tried gnupg.conf no-allow-external-cache option, which causes a different pinentry dialog without the checkbox, but then the private key password is not accepted. Is there a way to remove or disable that checkbox in the pinentry dialog? My system: MX-linux 18.3_x64 December 15 2017 base: Debian GNU/Linux 9 (stretch) gpg (GnuPG) 2.1.18