Antonym is defined as word with opposite meaning to another
word. Since 'stanza' is an arrangement of a certain number of lines, usually four or more,sometimes having a fixed length, meter, or rhyme scheme, forming a division of a poem, it does not have an opposite word, or an antonym.
However, if we look at poetry and prose as two opposing literary genres,
antonym of stanza could be a division of text, 'passage' or 'paragraph'.
Synonyms of stanza are verse and strophe.