Sybase ASE doing showplan for all requests
SET SHOWPLAN [ON|OFF] is a session-level command in Sybase ASE, it's not a server-level setting (if it was a server-level setting you'd be able to alter it via sp_configure). Can you run wireshark (or something similar) on the packets being sent from the JDBC client to see if it's setting showplan on as part of the session initialization?

That said - showing the plan should not affect database CPU or memory usage, the plan is generated by the query optimiser anyway. However, showing the plan will increase network utilization.