Could try the "Blasting Sands" or whatever spell that destroys resource tiles. I don't know if works on roads. Or try fortifying over the area?

Roads do block city placement, it is the game being stupid.

Neither Blistering Sands nor fortifying clears them.

Terraforming needs a overhaul atm anyway. Forrests should not be harder to clear than mountains! 

Is there a way to clear forest?

Blistering sands (Earth spell, Level 5 iirc) clears Forests, City Ruins & Resources. It also turns the tile to sand, not sure what that means, maybe diff monster spawns on it? The Water spell that comes later Forsaken Lands? does  bugger all as far a I can tell, except turn the tile wintery.

Not tried BS on Notable Locs like inns though, could be good for a laugh zapping a level 5 just inside a non-aggo's NPC's territory.