If a company has a "no violence" policy could that override a person's common law right to self defence?
Some kinds of companies (e.g. freight shipping companies and banks) often do have those policies. The real issue is not whether those policies are permitted, but what the consequences are for breaking them.
The fact that a company forbids its employees from exercising a legal right doesn't mean that the employee ceases to have that legal right. It simply means that if the employee exercises that legal right, then the employee has breached the contract and may suffer the consequences for breaching that contract.
Violations of those policies are grounds for termination from employment, and this would probably not be void as a matter of public policy.
For an employee at will this is really pretty meaningless, although it could conceivably affect unemployment benefit eligibility. But, for a unionized or civil service employee who can only be fired for cause, this is a big deal.
But, in theory, a company policy does not impact the tort liability or the criminal liability of the individual engaging in legally privileged self-defense to anyone. This is because two people can't contractually change their legal duties to third parties with whom they are not in privity (i.e. with whom they do not have a contractual relationship). And two people also can't contractually change the terms of a country's penal laws.
The policy may be a defense of the company from vicarious liability for the employee's use of force in violation of the policy that gives rise to civil liability for the employee because the grounds for authorizing self-defense were not present.
If the employee using force did so wrongfully and was sued for negligence rather than battery, the existence of the company policy might also go to the issue of whether the employee was acting negligently since a reasonable person in the employee's shoes might have been less likely to wrongfully use force in purported self-defense if there was such a policy than if there was not such a policy (and instead there might arguably have been a legal fiduciary duty as an agent to protect the property and workers of the principal in the absence of the policy).
As a delivery driver, someone might try to rob your vehicle. And you might have the legal right to prevent that with violence. So your company says you shouldn't do this, but walk away. You have the choice between self defense and walking away and having your vehicle stolen. In that case, you do as you're told and walk away.
On the other hand, someone might threaten you with a knife and you don't see any way to escape, so you have the choice between self defense or getting hurt. In that case you use self defence.