Can a court deny a motion for sanctions against defendants in a civil case?

Santos contends that the district court should have granted his motion for sanctions because defendants raised frivolous affirmative defenses in their answer to his complaint. The district court concluded that although three of the ten affirmative defenses in defendants' answer had no legal relevance to the civil rights claims in the complaint, the answer as a whole was otherwise appropriate. We have previously upheld a district court's denial of sanctions under similar circumstances, see Partington v. Gedan, 961 F.2d 852, 866 (9th Cir.1992), and we find no abuse of discretion here.