No. Messages are not signed. The connection filebeat->logstash is secured using TLS (same technology used for https). Messages are not signed (plus for signing you'd need a private key and verify using a public key) at all.
Client certificates MUST be used with TLS if you don't want all possible clients to send data to logstash. See "Client-authenticated TLS handshake" on TLS wikipedia article. For this to work, servers must be configured to require client authentication in first place. This is currently not possible with logstash.