He cannot block it. They are not trading the SPC to another tramthey are trading his exclusive negotiation rights, which are two completely different things. An NMC protects the player from having their contract traded to another team. There is a period before UFA where teams have exclusive negotiation rights and players exclusive rights can be traded within that period. The SPC is technically done 5 days before July 1st.
There is absolutely no concept of "trading" negotiating rights separate from trading the player.
The only "angle" the Leafs could theoretically try, is one they apparently tried ~15 years ago with Mats Sundin & Montreal... where Toronto agreed to let Montreal call up Sundin, with the understanding that there was a deal in place between Montreal & Toronto if Montreal was able to convince Sundin to sign.
Sort of similar to how Zack Hyman's deal went down, where Hyman was given permission to speak to a number of teams, with the Leafs hopign they would then come to Toronto asking for Toronto to facilitate an 8-year deal.
With Mitch, it's a bit different. It's not immediately that Mitch even wants a long term deal. There's a reasonable argument to suggest that he'd be better served with a 2 or 3 year deal and go back to UFA with a $115m+ cap.
If going long term, it may make sense to look at 6 years, so that he can still obtain another deal with term at age 34.
Ultimately, I think the hope for Toronto has to be around Mitch & his representation getting nervous. Let's say Toronto strikes a deal with LA... so Mitch gets the call from Ken Holland saying "we want you, here's $13m x however many years you want... but if you say no to this we cannot promise this offer is still going to be on the table come July 1 at 1pm, as we need to make decisions"... maybe he gets nervous and says to himself "this ticks all the boxes", and agrees.
"LA" does this because knowledge is power, and they can go into July 1 with certainty as to how much cap space they have to play with, and one massive need already filled.
On the flip side, there's also an argument to say that there might be even more money available on UFA Day as teams get desperate... maybe that $13m becomes $14m or $15m. I'd wager it wont come from a contender at those levels, so Mitch will have to decide what his priorities are.
If it's making sure he doesn't leave a cent on the table, then not much the Leafs can do. If it's wanting to assert control and be presented with ~10 offers with which to chose, again not much the Leafs can do. If he decides he really wants to be in a specific place (or two), and wants to ensure he's "properly paid" while going there, it's likely in his best interest to work with the Leafs so that that team can make it happen.
He'll also have a ton of information as Darren Ferris has the 2 big dogs in the UFA pool, Marner and Bennett.
