Refuse service?

Just finished a closing CSD shift, but during my shift had just 1 single negative interaction.

A lady came up to the CSD and was quite huffy and puffy, complaining that the queue was so long and she had been queuing for 15 minutes (no way it was this long, deluded exaggeration).

I told her I was by myself because my colleague was currently taking her break and yes, unfortunately queues suck (smile and lighthearted, try to prevent aggression).

She put a massive bag onto the desk, full of clothing and said, "refund all this, now, I don't have time for this stupid waste of time".

Okay, now I wanted to tell her to fuck off, but, I'm a lovely person, so I told her I would need a receipt, and I would see what I could do. She proceeds to start ranting angrily (and loudly) about how she shouldn't have had to pay for full price clothing when 2 days after she bought them the big 25% F+F event started, and said it was a conspiracy against her and Tesco were purposefully targeting after she had spent £400 on F+F. She asked if I was morally corrupt (because I dared to work for Tesco, who are so evil, obviously).

At this point my colleague got back from break, and I asked her to call a SL over, and when they arrived I asked them for assistance in the situation. They, thankfully, told me it's my break and they would sort it out, so I got out of there without any hesitation.

I've been told I can refuse service to any customers, but that feels so intimidating, especially when I'm on my own. What should I have done differently, and was this a situation where I could have told them I would not serve them? There was a big queue behind her, and I was on my own to start with, so I couldn't really walk away without abandoning the CSD.

Thanks for any advice.