I have very few hard and fast rules in my dojo, however, two that I do have are:
1) No "horseplay"
2) No practicing of any other fighting styles on the mats before, during or after class.
By "horseplay" I originally meant the general goofing off behavior that the children would normally engage in. However, recently I have had to remind some of the "adults" in my dojo of the rule. Sigh...
While I do not have anything against other self-defense styles, we are an Aikido Dojo strictly, thus Tai Chi, Muay Thai, Kenpo, etc. have no place on our mats except by way of demonstration by qualified competent instructors in those arts.
Past these two rules, I remind my students to not attempt an attack for which they are not ready with the proper ukemi. Too often some students who "resist" a technique either do not know how to fall properly when the nage gets it "right", or they think they are just being "real".
In the former instance, if the student does not know the ukemi or is afraid of it, I ask and instruct them to learn to take the fall rather than trying to fight it. It becomes Nage's duty then to guide Uke into the fall safely, not take advantage of Uke and "throw".
In the latter case, I often find Uke "pulls" their attack at the last possible second in their attempt to make it "real". I then ask them if they were really going to connect. Many times I will have them attack me exactly the same way they did their partner. This gives me a feel for what they are actually doing and to provide the appropriate feed-back. On a few occasions, I have simply stood there only to have their "real" attack miss me completely. Usually they do not repeat this particular error.
As an instructor, however, it is your duty to treat all your students with respect and fairness, even the "difficult" ones.