I think we actually have too little information to answer this in detail. There are tons of ways to resolve it based on the information given, and dice don't even need to be involved for many of them.
The player could just say he asks the person to stop, and you could RP the guy looking back and either stopping or going depending on the nature of the PC (likely stopping for Michael or Thomas, hoofing it for Molly or Harry).
If the NPC has a partial inclination to stop anyway then it won't even matter what the player does, almost anything will keep him from leaving.
If dice need to be involved you don't need have to think of it as block. The exchange can be thought of as a contest instead. They are practically the same thing in this situation, and since its not like it is actual combat the simpler one is just that, simpler:
If the PC doesn't get physical you could do a contest of Intimidation (threaten him to stop), Presence (order him to stop), or Rapport (beg him to stop) vs. Empathy or Discipline. The NPC could even use Conviction if he is leaving for a reason that is extremely important to him. e.g. Commander Vimes needing to get home in time to read his sons favorite bedtime story exactly at 6pm sharp (If you don't get this reference, shame on you, everyone should read Terry Pratchett).
If the PC does get physical then a contest still works. Might (chest bump), Athletics (get in his way and stay in his way, but without pushing backwards) or Fists (hold in place or block with arms) vs. Athletics, Fists, or Might depending on the NPCs response. For instance he could try to dodge around a chest bump (athletics) or shove an athletics attempt to the side (fists).
No matter which option is chosen, just roll a contest and see who wins. If the PC wins the guy stays for a number of exchanges equal to the shift, if the NPC wins then he gets away. If there is a tie the NPC gets away but the PC gains some minor bit of information about him that will help track him down later, like maybe a matchbook from a bar falls out of the NPCs pocket as he dodges around the PC.
There is no need to make this more complex than it really is.