How to suppress activation procedure output from the error log?
Seems like you have an activation procedure that explicitly outputs the string y (probably using print but possibly raiserror, select, or output).

Why don't you change the procedure so that it doesn't have any output? Output shouldn't be needed when run through service broker. Or if the procedure is also sometimes run interactively, and output is required in that case, you could require a param for output (or create a wrapper procedure and call that instead).

There is no trace flag to suppress activation procedure output from the error log. This was a design decision to assist in troubleshooting activation procedures (since they are not typically run interactively).