My kids never cheered HS but, do you all do mandatory parent meetings before tryouts? This is where I think coaches have an opportunity to outline the rules, talk about safety, leadership, expectations and have the parents sign a contract, as well. Tell them what the consequences are and let them know you understand traffic and bad moods happen but, their athlete will be there on time and act appropriately or be disciplined . I know some parents can be intimidating, that's their plan but, this is where you can stand firm and steely eyed and say, "Mr/Mrs Jones, you were in the meeting, signed a contract of what the expectations are and your child is not abiding. Rules are put into place for the safety of our kids and for the integrity of the program and school. Your child can sit out for 2 quarters as stated in the contract or you may remove them from the team if you feel they are no longer capable of following the rules you both agreed to and signed prior to tryouts. What would you like to do?" Keep eye contact, let the parent make the choice. If they don't answer, I would ask them if you need to repeat the options. If not, let them know their child will be sitting out 2 quarters or they are welcome to come and remove them from the team permanently and walk away. If they say, "Don't walk away from me.", turn around and ask them what you need to clarify for them and, if nothing, you have responsibilities with the team and leave. Ultimately, your AD is the difference, if they support you, then word will get out and parents will behave.