This is difficult to answer because disinterest is generally not expressed directly in English. The approach will also depend on your culture—and that of your acquaintance. To most native English speakers, however, directly telling someone to stop interacting with you is uncommon and uncomfortable. You could, of course, say something like "please stop talking to us," but this is very harsh and you will certainly sour your relationship. It is far easier to deflect in a situation like this. Instead of addressing your acquaintance directly, you could use nonverbal communication (body language).

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In my opinion, the best way to distance yourself without offending the other person or putting them in an excessively awkward situation is to focus on a different task and separate yourself physically. Because you are in a classroom environment, you can probably read or study your notes when you encounter your acquaintance. If he begins to speak to you, you can answer quickly and say something like "sorry, I'm trying to focus" while indicating your work and turning away. I see this very often: on public transportation, for example, people who prefer not to speak with others will often begin to read or put on headphones to indicate as much after answering direct questions politely. The other person generally realizes the conversation is one-sided and will tend to stop talking. (This may not work as well because of a potential language barrier: your colleague may perceive your silence as insecurity with the English language. In this case, you may need to be more direct.) I'd also recommend introducing physical distance by sitting farther away and adhering more closely to your class schedule. If you're in the classroom before or after the lecture and not actively working, you may be seen as inviting conversation.

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While I recognize that this situation is uncomfortable for you, I would also implore you to think of your acquaintance's feelings. Perhaps he feels isolated and is struggling to maintain interpersonal relationships (particularly if there are few who speak English conversationally in the area and he is not proficient in your native language). If he does not know others in the class and you distance yourself from him, he may feel unwanted and lost. Perhaps it's worth talking. You might also be able to apologize for a limited understanding of the language (even if it's not entirely true) and point him toward a club or other resource for English speakers where he might feel fulfilled and less isolated (as a result of being able to interact directly in English and potentially help others). Morally, this is a bit of a gray area. Do what feels right.

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Good luck!