A better boundary (for me) is to make plans for myself and not change them because Mr. A does XYZ. For example, I have plans after work tomorrow night. If Mr. A calls between now and then and asks me to switch up my schedule, I'll decline. I already have great plans! I think I can stick to that.
Sounds like a plan. A good one.
Gardener
"My soul, be satisfied with flowers, With fruit, with weeds even; but gather them In the one garden you may call your own." Cyrano deBergerac