Blimey.
This has been an interested thread. At first when I read the card I thought, 'ah, a clever fellow, there is a message here' although it was quite obvious his way of translating this was - off the wall to say the least!
He has tried to present his feelings 'shrouded' in cyrptology. This isn't uncommon, types of autism manifest themselves in people masking what they say with riddles, jokes, etc.
Then you mentioned that he had done this before, to other girls. That makes it a different matter. He may be lonely, he may be funny, he may be worthy of a chance, but he is bordering on stalker.
A quick pointer about the law. The police will do nothing. What you have as evidence won't stand up as anything, regardless of who it comes from and so on.
However. Your Associated Diaries, as your employer, owes you a duty of care as their employee. As part of the Employments Right Act, you have a right to work in a workplace free from numerous quantifiable dangers. This includes harrasement, abuse, and so on.
If you percieve that this threat exists, and you take it to your managers they MUST respond, or they have broken the law. Simple as that.
Look at what will happen to KFC in the coming weeks following last weekends events.
How they choose to respond is up to them and you to decide.
The most effective way would be to ban him, and it seems your store isn't adverse to that. Or they may insist if he is to shop at ASDA he must be served by a male operator. If they don't thats where the police will come in.
Anyway, I have rambled here. My point is that you shouldn't have to change your work or your schedule around one bloke. The shop should take care of you, but taking 'care' of him. So to speak.
Good Luck in getting this resolved and remember - he's the one at fault here, not you!