This is a revision after the first round of peer review.

Background and Discussion are expanded. More detailed discussion of the limitations of blacklisting.

New supplementary figures extend the results in the main text to three referees as opposed to two, and lognormal distribution of quality as opposed to normal.

A new type of referee is included who rejects papers that are too different from their own.

Correction to previous result: bypassing referees is more effective than previously reported.