Sanji kicks down stone walls with no problems but he can't break a steel net. That's a bit awkward logically.
However, in defense of such logic, walls are not as elastic as steel is, so steel should be tougher to break.
In criticism, the entire mass of a stone wall is very strong against wear and tear. In earthquakes, stone moves back and forth but return to its original shape. So the manga isn't making sense.
In defense, in earthquakes, stone tends to break under pressure, so walls are reinforced with steel for its strength and elasticity. That makes steel > rock.
However, the width of each link in the net is miniscule compared to the girth of a rock wall, so it should be relatively easy to tear. It's like spider webs are stronger than steel for their width, but they're still easy to break by our standards.
Well, Sanji isn't kicking the net, and he doesn't use his hands to attack.
Sanji could use his legs to tear through the net.
He could but he wasn't given time. Caimie saved him too quickly.