For a resort of its size, St Martin de Belleville has a good number of bars and pubs though no nightclub.
One of the favourite après-ski/evening destinations is the relaxed and cosy Pourquoi Pas (P Bar), with its open fire, and the bar typically provides live music three times a week with an excellent lively atmosphere.
For some après-ski immediately off the slopes then Lodji Hotel or Le Dahlia has to be a great choice; situated at the front-de-neige, both are great places to lounge in a deck chair at the end of a hard days skiing with a glass of something cold (or a vin chaud) in hand. Lodji Hotel has a great south-facing terrace for some late afternoon sun.
Finally, L'Art B at the foot of the St Martin drag-lift which has a much more French feel to it.