I think stinking up a bus is more offensive to a person than obesity costs.

Sounds like you've never had to pay taxes.

But by your reasoning guns should be illegal since they will always cause mass shootings, murders and associated costs.

No by my reasoning much like in Switzerland I'd make gun ownership for Whites mandatory but also fund shooting ranges and other methods that help people use guns in an efficient manner. As usual though every single time you talk with a libertarian you're only one or two sentences away from terrible comparison or loony hypothetical usually involving guns.

Where do you draw the line in limiting freedoms?

I draw the line as far away from wherever libertarians say it is and start from there. It's actually a pretty good technique. With foreign policy I just go to Max Boot or Bill Kristol's twitter and figure out the right thing to do by doing the opposite of what they say and it's the same with libertarians like you. Always do the opposite.