1. A computer-implemented method comprising: constructing an association graph that includes one or more terms as vertices linked by edges, which edges have respective edge scores that represent respective strengths of association of the respective vertices linked thereby; receiving from a user a search query that comprises one or more of the terms of the association graph; responsively to the search query, generating, using the association graph, refinement options that comprise respective terms of the association graph, and presenting information to the user, including the refinement options for optional addition to the search query; identifying a portion of the refinement options, and corresponding terms, that the user did not select; using the association graph, assigning respective ranking scores to search results generated responsively to the search query, including lowering one or more of the ranking scores of the respective search results that are characterized by the non-selected portion of the refinement options, by damping the edge score of an edge of the association graph between two of the vertices respectively representing two of the non-selected terms; ranking the search results according to the ranking scores; and presenting the ranked search results to the user.