How to stop keychain / Safari from storing identity preferences?

I have several digital certificates installed in my computer and stored on my keychain. Certain websites (normally from Spanish government) require user identification through these certificates. Whenever I log in a website or service using any of my certificates, that is stored as an identity preference in the keychain. The same certificate is used again without asking when I log in the same site again. This is inconvenient, because if I need to use a different certificate, I must go to keychain, delete the identity preference and restart Safari in order to be offered the choice of certificate again.

Does anyone know how to stop keychain or Safari from storing identity preferences? Do you know of any workaround?

Many thanks.