ASM / WAF : block request containing certain string?

I have added as much XSS blocking to a policy as possible. A request containing onmouseover or onclick or .... ="alert('hello')" is blocked fine.

But when it's coded like onmouseover or onclick or .... ="self['\x....... the ASM accepts this as valid.

Can I block a request with this parameter value?

How do I achieve this?