In a timed game you can play them at their own game. Destroy the open half of their fleet and then turn around and just move around on your side of the board well out of range and let them cone to the realisation that if they don't move you get a victory.
Its not fun but I did that against someone fortressing and they complained I wasn't playing fair by not attacking his fortress, they quickly came to the realisation and the point I was proving. While they still occasionally fortress they now do it to stall before they break
I love opponents who think it's slow playing if you don't blindly rush your ships into their firing arcs. This is a dogfight, not an Old West gunfight in the middle of the street.
