How to Replace Hash Match (Partial Aggregate) with Stream Aggregate
Not a lot of information here, but to directly answer the question that was asked: you can use the ORDER GROUP query hint. From the documentation:


  { HASH | ORDER } GROUP
  
  Specifies that aggregations described in the
  GROUP BY, or DISTINCT clause of the query should use hashing or
  ordering.


That query hint may help you verify that a stream aggregate is a better choice than the hash aggregate. A few things to keep in mind are that it applies to the whole plan and it will continue to take effect even if there's an index change which means that an explicit sort is required.