Wouldn't his refusal nullify the no trade clause?
No. His no trade list is what it is. If he's traded to a team not on the list he could potentially refuse to report, which would end up with him being suspended. The acquiring team could also try to move to terminate his contract.
The whole reason we're having to talk about this, though, is that other teams don't want to have to deal with this shit. If a team thinks a guy might not report, they have no reason to trade for him and then have to deal with the ensuing headache. It's just not practical. Trouba was a headache for Winnipeg and now he's a headache for us.
This was my point last night about why it's easy to trade him on paper, but not necessarily in reality. On paper, we can trade him to 16 teams, no questions asked. The reality is that Trouba doesn't want to play for many of those teams and might not want to play for ANY of them, and in turn there are very, very few teams that would want to try to trade for him (or claim him on waivers or whatever).
Across all sports, we see players force their way to various places and prevent themselves from going various places, basically by doing nothing more than making their wishes public knowledge. This is one of those things.