the best way to avoid having people walk all over you like that is to make them fear you. My last job pissed me off constantly and was a constant deluge of bulls*** but I'd always start drinking 1-2 hours before the end of a shift and most people knew about it but let it happen because those hours were our busiest and I was actually more functional while drinking because I was able to have fun and be calm and relaxed instead of pissed off, volatile and ready to snap.
so yeah I would tell them you can't work that shift because you have made other plans for that time, which you did because you had already informed them you were unable to work that shift and had no reason to keep your schedule free on account of informing them you were unable to work that shift (and phrase it like that as well. Being reiterative for the sake of hammering a point across comes off as bizarre. Bizarre might be a symptom of crazy. Crazy is often not-to-be-f***ed-with. Lead them down that logic-trail and they will know that certain boundaries must not be crossed and as long as you are a hard, efficient worker who does not show signs of taking too much advantage over the fact that they won't f*** with you, you're good)
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I never said anything about banging her. Also, I wouldn't bang her. -LusterSoldier (on a fat pageant mom whose daughter chugs Mt. Dew)