What can I do about someone who makes a lot of assumptions about me?

The primary response is frustration and the desire to correct their assumption. Often, people with assumption disorder are having some deeper cognitive issues going on like attentional defecits, poor critical thinking, narcissim or entitlement, delusional thinking, or just plain lacking in critical thinking skills. With this in mind, you are best to be like Mr. T and pity the fool as their bold assumptions indicate an underlying disorder. If you cannot simply shake your head and walk away ghosting this person forever, then learn to ignore their assumptions by seeing them as silly rather than as an attack or insult on your self-concept.