Vidaloco has the right idea. If you don't want to use a whip, (although it won't hurt her if you do) then when ever she acts up, you need to be the crazy lady and scream, yell, wave your arms at her,stomp your feet at her, and let her think she is going to DIE!!!!! (for 10 seconds, then stop and act normal)
Yearlings can be pretty silly at times, but they are also children of a sort who are learning about the world. What may have been cute as a foal, or is adorable now, will NOT be when she is 1200lbs. She needs to learn manners now, not later. She needs to be disciplined instantly for any misbehavior she exhibits towards you. This does not mean beating her, or "being mean". Discipline is not mean. It is what a GOOD parent does to teach their "children".