Syslog-ng 3.5.6 is the latest version currently offered by Debian stable (jessie).
I'm not sure how to blacklist ciphers with this version.
Newer versions of Syslog-ng seem to offer a more sophisticated cipher-suite() and ssl-options() directive.
I'm not sure how to interpret what is written in the Syslog-ng 3.5 Documentation: cipher-suite()
Ideally I'd like to disable TLSv1.0 and also RC4 ciphers for syslog-ng 3.5