We intentionally set the contract sizes of BitMEX products at low values to encourage traders both large and small to trade on BitMEX. However, some traders abuse this and spam the orderbook with lots of small orders.
The following change was made effective on 10 June 2016 12:00 UTC.
If an account has too many open orders with a gross value of less than 0.01 XBT, they will get marked as a spam account and subsequent small orders will be hidden. As always, our goal is to create the best possible trading experience, and this change helps us to achieve this goal.
Update: on June 10 2017 the gross value definition for spam orders was updated from 0.01 XBT to 0.005 XBT.