OpenSSL – Prefer Server Ciphers

I’ve got a running OpenSSL::SSL::SSLServer. But even when I define the
ciphers with an SSLContext, the clients can still choose their own
cipher, as the server will follow their preferences.

Does someone know how to enable the SSL_OP_CIPHER_SERVER_PREFERENCE flag
with SSLContext, so that the SSLServer instance only accepts ciphers
defined on the server side?

Thank you for any advice!

Best regards