Ahhhh, I had similar with my mum. She was always reliant on me to top-up whenever she had prioritised cigarette and takeaway poorly planned her finances.
It was the same with the paying back too - what started as a significant amount of money returned to me in dribs and drabs that just evaporated as soon as it was received. My bank account was actually going down.
It got to the point where she was just being a CF asking every single week/fortnight. I had to have a very frank, and even exaggerated, discussion about my finances and how they could no longer stretch to cover her. I told her that under no circumstances could I allow her to continue impacting upon my children's needs (yep, played that card) and how any potential savings were being locked away in an extremely tight bank account / ISA that would cost me a kidney to make an early withdrawal.
I didn't ask her to sit down and have a nice little chat, I just waited for the nest request and then gave it to her as fact "OK, yes, you can borrow it this week, I'll bring it to you on Tuesday, and I really do have to say that this is the LAST time I can help you out because of..." Then on Tuesday I reiterated it "remember I said on the phone about xyz reasons? Yes, they stand. Today is the last day I can help you out".
I thin told her I wasn't abandoning her in relation to FOOD. I explained that I could always put it put that on my very expensive 50% interest credit card if she ever truly required it. 
It worked out great and she's been a lot better this last year. I guess she realises she can't just go get the takeaway and then ask me for the cash to cover it.
I did have the added benefit of not working at the time though, and I think she took it seriously that the cash cow had indeed run dry.