A facility owner does have the right to deny entry (especially in the case of matches) to individuals, as the facility is their private property (the rules are different in the case of a publically owned faciliy). We had a situation in our area where a facility owner had banned a person from her property, and AKC confirmed that while banned that person could not attend events there (it's entirely up to the person owning the property).
As to the grumbling/growling, it doesn't really matter why the dog is doing it, it just needs to be stopped. It's a quick way to loose points (any noise generally means points off), get excused (or even DQ'ed depending on the judges interpretation) during a trial, and it WILL intimidate/distract other dogs (especially soft dogs) and handlers. You don't want your dog to ruin someone else's chance at a qualifying score
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I'd go back to basics with this dog...start with tiny increments of quiet stay time, and build back up to the full time. You want to cut the time down to what she can handle without talking. The grumbling could very well be a sign of insecurity with the exercise (many trainers and handlers increase time and distance so quickly on the stays that they create this), and backing up to a point where she's comfortable will help rebuild her confidence in exactly how she should be doing the exercise.