Please find attachment that may be helpful

The 1st paper on this compound showed that it had time-dependent inhibition which was prevented by dithiothreitol. A later paper found that STAT3 was alkylated on cysteine residues by stattic, supporting a covalent mechanism (in the absence of competing thiols, presumably). Covalent inhibition is not always irreversible, however, so this doesn't fully answer your question.

http://www.ncbi.nlm.nih.gov/pubmed/23810499