No, you go "I'm sorry but we're paying you for quality work and the dev you assigned us is clearly a junior dev. If you do not have the capacity to do this job then we would have preferred you simply stated this up front" and then you don't "withhold payment", you make it a contract condition and you terminate the contract and find someone else.

It has nothing to do with developers you _like_, but with developers who are going to deliver what has to be delivered in the timeframe set out in the contract. If an interview shows they're not going to be able to, then the company did not provide you with developer to do the work, they provided you with someone who can't do the work.