We are a local Housing Association and are in the process of considering different ways we can award good tenants. One of our ideas is to give them a one-off payment in either cash or vouchers (approx £500).

I think this would be considered as capital within their claims for benefit. However, it would be preferable if it could be disregarded as we do not want to affect those tenants who already have capital as the extra money may affect their entitlement.

Does anybody know if this could be disregarded and if so under which regulation?

I do not see a way to to disregard a monetary gift,or anything which could be converted into cash (like premium bonds). But vouchers should be OK (assuming you mean retail vouchers that can only be spent on goods from that retailer).