Hello I am a solicitor with 20 years experience. I will try to answer this.
Writing "not used" sounds like a risky thing to do that could cause ambiguity. It is much better to removed the clause entirely. If this is difficult (eg if using a standard pre-printed contract) then you could cross out the offending clause, but make sure all alterations are signed by all parties to the contract at the point wheree the alteration is made
and not justcat the bottom ofcthe contract.