Random Acts of Violence - S3-E13
Other mistake: Just after they arrest the Janitor, they are walking out of the building past the employee's. The girl sitting at her chair is not actually typing on her keyboard, her fingers never even press on the keys and her screen-saver is displayed on the screen. (00:37:15)
Answer: He doesn't have the authority to arrest him - when anyone is arrested in the show, a police officer does it. Also he tells Ty that he is going to be submitting his case findings to Ty's insurance company, who would no doubt contact the IRS, who then would have him arrested for fraud etc.