The phrase "drop in on" typically means to visit someone unexpectedly, but there are a variety of antonyms that express the opposite sentiment. For instance, if you're looking to avoid a sudden visit or interruption, you might opt for the antonyms "stay away from," "avoid," or "bypass." Alternatively, you might use phrases like "schedule an appointment with" or "make plans to visit" to be more intentional about your visitation. Other options might include "book in advance," "request a meeting," or "plan ahead." Ultimately, the choice of antonym depends on the context and your desired level of formality or intimacy in your interactions with others.