I know I should be able to work this out for myself, but I have got it wrong a couple of times and it's driving me insane. 

I have a dropbox and my phone is linked to it. It's full, and I have tried removing files from the PC version of dropbox because it's so much faster than from my phone or via the web. Problem is, then the phone just re-uploads it, and it's full again. 

I tried removing some from the phone and they syncd back from the PC version. 

I have camera uploads enabled.