In the settings of a macro group you find a Disabled on this Mac checkbox. So, you could put that macro into a special group, and disable the group on every Mac but one.
For example this group is only enabled on my main computer, not on the others:
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Not sure if with “sharing stored macros” you mean that you store macro files on Dropbox and then import them on different Macs, or if you mean the built-in Sync of KM.
In case you have missed the latter, you can activate the Sync in KM’s preferences:
I hope you don't mind that I have revised your topic title to better reflect the question you have asked.
FROM: Specify computer?
TO: How Do I Limit Macro to One Mac?
This will greatly help you attract more experienced users to help solve your problem, and will help future readers find your question, and the solution.