My initial suggestion was to be, "treat his actions realistically and enforce appropriate consequences, both social and societal." But the edit you've added shows you've already done that, and it doesn't seem to have had much of an impact.

Given that, the step to take next is to ask him directly not to be a goof-ball. Use the example of the last chronicle as a guidepost of what you don't want: you're hoping to run a "serious survival story" and wacky madcaps and cartoon slashers aren't appropriate. If he can handle that, great; if not, express your regrets and invite him to the next game that can handle a little more dedicated silliness.

You might find some useful information in the answers to this question as well.

My initial suggestion was to be, "treat his actions realistically and enforce appropriate consequences, both social and societal." But the edit you've added shows you've already done that, and it doesn't seem to have had much of an impact.

Given that, the step to take next is to ask him directly not to be a goof-ball. Use the example of the last chronicle as a guidepost of what you don't want: you're hoping to run a "serious survival story" and wacky madcaps and cartoon slashers aren't appropriate. If he can handle that, great; if not, express your regrets and invite him to the next game that can handle a little more dedicated silliness.

You might find some useful information in the answers to this question as well.