One encounters the phrase has no authority to and shall not in the context of agency:
The Contractor has no authority to, and shall not, act as agent for or on behalf of Client or represent or bind it in any manner.
Consultant understands and recognizes that he is not an agent of the Company and has no authority to and shall not bind, represent or speak for the Company for any purpose whatsoever.
But forbidding someone to act as agent is broadly equivalent to not authorizing them to act as agent, so using both would be redundant. Of the two, using shall not is the better option, as it’s clearer you’d have a remedy for breach if someone nevertheless holds themselves out as being your agent. But a sensible starting point would be to say, as in the second example above, that whoever it is isn’t an agent of the company.