Postfix:
Disabling certain versions of SSL works like this in Postfix:
In your /etc/postfix/main.cf add or modify the following config parameter like so:
smtpd_tls_protocols=!SSLv2,!SSLv3
If you are using mandatory TLS you’ll want to set this instead:
smtpd_tls_mandatory_protocols=!SSLv2,!SSLv3
These should be fairly self-explanatory, but for further detail read the Postfix configuration parameters documentation.
Do not forget to restart Postfix!
Dovecot:
In dovecot, add the following to your configuration:
ssl_protocols = !SSLv2 !SSLv3
…and restart Dovecot. If you use a version of Dovecot older than 2.1, upgrade and then do the above.