What is the appropriate sanction for a district court's failure to allow a defense witness to testify?

Lopez asserts that the district court abused its discretion in refusing to allow a defense witness to testify. The witness had remained in the courtroom during the presentation of the prosecution's case in violation of an exclusion order. We review the imposition of a sanction for a violation of Rule 615 of the Federal Rules of Evidence for abuse of discretion. See United States v. Avila-Macias, 577 F.2d 1384, 1389 (9th Cir.1978) ("The appropriate sanction for violation of a witness exclusionary rule is a matter which lies within the sound discretion of the trial court.").