According to Yahoo's Jeff Passan

The ban is reportedly a result of Boston exceeding a $300,000 signing bonus limit imposed on them for last year's signing period, after the Sox exceeded it's limits by dropping $62 million in 2014 on top prospect Yoan Moncada.

As the latest free agent signing period gets underway, Boston must not only sit out from signing potential prospects but must also watch as five of their own prospects head back on the market.

The five players whose contracts were voided include outfielders Albert Guaimaro and Simon Muzziotti, right-handed pitcher Cesar Gonzalez and infielders Eduardo Torrealba and Antonio Pinero.