Can I cancel a direct debit to me without telling the other person?

My parents sent me £300 a month to help with rent while I was at university. When I finished and got a job I asked them to stop but my dad said he wanted to keep paying for a bit as my 1st job was poorly paid and my rent was very high. Its now 3 years later and they're still sending and I'd like them to stop as I'm earning a bit better and feel guilty. I don't know if they know they've been sending it and can't really afford to pay it all back tbh. They obviously don't need it or they would have said/noticed so I wondered how I cancel it without them being notified. I'm with nationwide - do I just ask in the bank, couldn't see a way to do it online...