To use a legal term,

Ask for "PROOF OF CAUSATION"

They HAVE to prove it was you, not assume it was you. and if there is no damage to the van that is clear.

worked for me about 10 years ago.

They can do what they like - it's their insurance. I would expect the insurer to tell both parties to do one.

There is no just no way that you're liable (unless you did hit the wall, are pretending you didn't and someone has proof up their sleeve). There is evidence you were there, which you don't deny, and evidence that the wall is damaged. That's it. Several other vehicles may have used the car park on the same day and the van you hired has been inspected and noted as having no damage. How could it have been you?

In terms of composing a reply, simply state that you did not hit the wall, that the van was undamaged, and that you won't be entering into further correspondence.

I repeat - have you left an area with vindictive neighbours or some dispute?

It doesn't matter - the hire company has confirmed that the van suffered no damage during the hire - end of negotiations!

Cease all communication with the hire company.

If they continue harassing you, make out an official complaint of harassment to the police and fill them in with copies of all the details and paperwork.

I can't quite see how accusing a customer of damaging a wall with an unmarked van is the easy option. The easy option is to ask the accuser to prove it, which they can't, and tell them to do one.