Carriage bolt/tag along hitch problems

This is the hitch for a tag - along which someone gave me. The seatpost collar is held together by two carriage bolts. One of the nuts won't loosen. Its not rusted.

Because its a carriage bolt I can't get a good hold on it. I have tried dremel-ing a groove to hold it with a screw driver but that doesn't work.

Any thoughts?

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Looks like there’s a slot below the domed head, does the bolt have a flat collar that slots into that to allow you to brace the bolt?

Well spotted. There is one on the bolt that works, but I can't see it on the other. In any case the bolt won't brace, and thats the problem

Looks fat enough for vice grips on the dome side?

They are about 5mm, and there is almost no edge to grasp onto. I have tried an adjustable spanner, which didn't work. Will try vice grips. Ta

Saw it off and replace the bolt?

ie saw through the middle? I thought of that but won't I have the same problem of not being able to brace the bolt?

Grab hold of the threaded end sticking out and hacksaw the bolt through the middle bit?

yes I think that would work.

Not sure I understand, you should be able to cut it without issue I thought

Yeah cut that shit off, use a bolt that has a head instead. Sounds like however the bolt should be braced in that collar has worn out.

@psg1ben I was being stupid. I thought that the collar had threads as well as the nut. Anyway, it didn't so I cut the fucker in the middle, as you and @midlife suggested. And with a dremmel, which I don't usually have a use for. So problem solved, inc sparks and noise. Now I'm having a negroni and eating crisps. Wins all round.

Thanks everyone