macbook aluminum started backing up to time capsule without asking I normally back up my mb to an external drive at home with time machine (it's been about 3 weeks since my last full backup).  Recently a co worker sent me a file by placing it on a time capsule we have at the office.  I grabbed the file and thought nothing of it.  

Today I noticed that the time machine icon was spinning like it does when preparing for a back up and after investigating I found that my mb was trying to back up to that time capsule.

I removed the drive from my list of backup locations, is there any way I can ensure this doesn't happen again?  Note that osx never asked me if I wanted to back up to the tc.
I don't have a Time Capsule so I'm not 100% sure this will solve your problem, but I use TimeMachineScheduler to ensure that Time Machine only runs when I want it to (generally on my not-at-work hours).