The best time to prune is Feb/March and you can prune it as hard as you like. We cut ours back to a stump about 4' high to move and now 4 years later it is 20' long and 10' high with interim pruning.
If the grapes are inedible then get rid of it, make wine or harvest the leaves for stuffed vine leaves - delicious!
There's a good website which gives info on which canes to cut out and what to do with what you keep etc
http://mtvernon.wsu.edu/frt_hort/grape_pruning _basics.htm