Unconfirmed reports also suggest more jobs could be on the line across the media owner's international network.
Wolrich joined CBS from Publicis Groupe last year.
Before joining CBS, Stark was an account manager at TDI Impact and prior to that a senior sales executive at news group.
A spokesman for CBS Outdoor refused to comment.
The departures follow the resignation of Andrew Oldham, former chief operating officer at CBS for the UK and Ireland, in May.