J.R.
I think you two main choices are this...1. get a fence, or 2. Make a boundry, maybe just some steaks with string tied between, maybe a border of the lawn ornaments, and show and tell her that she has to stay on this side of the line, then immediately when she disobeys/ingores you, take her in the house for a time out. Only take her back out after she calms down (you know she will have a fit) and repeat that "we stay in our own yard inside the lines". Repeat this every time, and soon she should learn that if she doesn't stay in "her" yard, she will not be outside at all.
I had to do this with my little blondie about wearing a hat outside so she wouldn't get sunburned...we would immediately go back inside if she took it off...a hat was the #1 requirement for being ouside. It only took about 3 return trips inside ( and the ensuing tantrums of hers on the floor) to get through to her that I was not kidding around. She will now get her hat and put it on if she is asking to go out.
Good Luck!
Jessie