Can a neighborhood in a city form a negative eruv techum so one would then need to intentionally make a techum to get into it?
Can it be done with an eruv chatzerot?
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No. The Mishna in the Gemoro Eruvin 81b that you cannot make a (positive) eruv techumim for another person without his knowledge because he loses something from it (viz. the ability to walk in in the opposite direction to the Eruv) as Rashi says
חוב הוא לו שמפסיד לצד האחר ושמא אינו נוח לו:
I infer that all the more so, one cannot make your postulated "negative eruv techum".