Mello gets one chance to come into custody peacefully, after that...

It'll only be by blunt means, at least? And anything worse than bruising will get healed up after, at HQ. If Mello merely resists without being a genuine threat then it'll be limited to grappling (I mean, it's still grappling in full plate), but worse than that -- i.e. shooting at them -- will get actual striking etc., though his sword will remain sheathed.

It's almost a pity they don't have stocks.