dismiss has 21 distinct synonyms spread across 6 of its 6 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
bar from attention or consideration
“She dismissed his advances”
stop associating with
“They dropped her after she had a child out of wedlock”
terminate the employment of; discharge from an office or position
“The boss fired his secretary today”
end one's encounter with somebody by causing or permitting the person to leave
“I was dismissed after I gave my report”
Synonyms, antonyms and the broader and narrower relations for dismiss come from Princeton University’s WordNet. Full sources and licences.