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A good question. The rules are not perfectly clear on this, but they do say "This phase ends when no player can or wants to acquire a city for a company", and that "wants to" sounds like buying cities is not mandatory.
On the other hand, "When it is a player's turn, he chooses a company for which he has at least one share and selects one of the cities in the bottom column (just like with shares.)" sounds like you don't have a choice.