Noun for a person, out of a group of many, who did not give their approval for something

I have information that serves as evidence of my company's illegal activities. I share that information with 10 others in my department. One of them goes to the press. That person is the whistleblower.

I have a form that I've filled out that needs approval before being submitted from 4 different bureaucrats. 3 give the form their rubber stamps of approval, but one does not. That person is the blank.?

Bonus: Is there a generic term for "a person who [...]"?

The person could be wilfully withholding their signature, or they could simply be on vacation and unaware that a signature is required.

In either case, and even with respect to things rather than people, a general term for this is bottleneck.

[Merriam-Webster]

noun
2 a : someone or something that retards or halts free movement and progress

transitive verb
: to slow or halt by causing a bottleneck

// All decisions must be approved by the committee, and this is where the company runs into bottlenecks.

So:

That person is the bottleneck.

The words can actually be used in a much greater number of situations than just that of somebody refusing to give a signature (or simply not having given a signature).

Some other possible contexts:

"Charlie, why don't I have your proposal on my desk? What's the bottleneck?"
"Donna, you know I'm waiting for everybody to agree to this deal. Who's the bottleneck?"
"Those guys are just a bunch of lazy bottlenecks."