WordNet records no synonym and no antonym for run-on sentence. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
an ungrammatical sentence in which two or more independent clauses are conjoined without a conjunction
Synonyms, antonyms and the broader and narrower relations for run-on sentence come from Princeton University’s WordNet. Full sources and licences.