Capping un-needed heating duct?

I'm amazed at how seemingly simple things can get complicated...

I have a heat register/duct that is in an outside wall--originally installed to heat what was formerly a garage, but is now a carport. (Long story...)

Aside from not meeting code and providing what looks to be a well traveled path for small varmints to get inside the crawl space, I want to terminate this duct for obvious reasons.

In the crawl, this duct is at the end of a long rectangle run--tree style with small ducts feeding various rooms. Second to last split already feeds the room where this duct is in the outside wall so the room has heat. I would like to get the duct out of the outside wall so I can seal and insulate better.

Problem is, the last split is complicated--there is almost a mini-plenum with a 90 deg take-off to feed the adjacent room. Attached on the side of this is the small straight section that extends another 20" or so before going up into the wall cavity.

First, I thought I could remove the sheet metal "daggers" at this split (I'm sure they have a proper name)--but I can only remove the one on the outside edge, since the inside one seems to be attached to the duct/plenum that feeds the adjacent room--the "dagger" has a "bottom" but no top folded over & seems immovable. Then I started thinking that I might do better if I could leave that section intact and cap a little farther on.

Should I just sawzall/snip the end of the duct just past the split and attach some sort of a cap on the stub end and leave the feed for the other room and the split intact?

Or?