There is the transfer service mentioned by Marcel. Mind however that you need to book it in advance, at the latest on Wednesday before the weekend you are planning to travel. There is no way to buy ticket on the day, even though usually there are free places. Also, you need to have a valid reservation with one of their accepted accommodation providers, they actually check with them and may cancel your booking.
Also there is a DolomitiBus service from Venice at around 7am, and back from Arabba to Venice at 16:18. This operated daily during Christmas period, and then on the weekends until 24th February. (I can't find any online reference)