You can flag a venue as closed, duplicate, or as having an incorrect address from the app. When in the app, head over to the listing page and tap on the title of the location. From there, tap the small 'report' in the upper right hand corner. A Superuser will review and approve your suggested merge. (If you're a Superuser, you can suggest merges from the Edit Venue view, and go through the merge queue for your town.)

You can also report duplicate venues on your computer by clicking "Report a problem" on the venue page!