Complaints should be brought directly to the head instructor of the dojo.
I should point out that our dojo is located in rural western Massachusetts where small towns are the rule and the largest city has a population of only 40,000 or so. There are only about 100,000 people in all of Berkshire County which covers roughly 800 square miles. As such, we may not experience the same problems that occur in dojos located in more densely populated areas.
Regarding the relationship between master and student; I was taught that the instructor is ultimately responsible for everything that happens on the mat. This is how I was brought up and how I run my school.
Because I accept full responsibility for what occurs on my mat, I have no trouble expecting my senior students to treat less experienced students with respect and consideration of their rank. Conversely, beginners and lower ranked students are told not to "test" higher ranked students by trying to resist the application of technique in practice. Resistance plays a part in practice of advanced students who have attained the necessary skill level in order to avoid injury whether uke or nage. Violence in any form on the mat is not tolerated.
Organizationally, the dojo is co-owned by my wife Mary and me. All classes are taught by either one of us. She is my most experienced student, having studied with me for over 15 years. We share common views regarding violence on the mat and our responsibility in providing our students with a challenging but safe environment in which to discover their Aikido. We believe that we have made our dojo into something special and unique (don't all school owners feel the same way?). We have avoided the cliques and power centers that tend to form in larger dojos and nurturing is far more prevalent than posturing on our mat. Perhaps the fact that our student body is about 65% female has something to do with this. Students with axes to grind tend to weed themselves out and move on to other things.
In large dojos with many instructors a formal system for handling complaints should be established. Perhaps incidents like the one that was aired on these boards can then be avoided.