Your problem here is you. You were booked. When someone sends you a double booking, hit "DECLINE" and they get an email right away saying you declined it.
Then you say "Hey I already had something in my schedule for that time, can't make it. You have access to my calendar why would you make meetings when I'm busy? That isn't very respectful."
If someone asks if a block of time is "real" always say yes. your problem is that apparently you have blocks of time that aren't real. if you only have real blocks of time in your schedule, nobody will guess which aren't or are real.
If someone asks you in person, have the backbone and willpower to say "Hey I'm already booked" and suggest another time.
You're the one causing your problems, but you're describing it as other people doing it to you.