This might sound silly but its true, there are many players of paradise island, who actually dont know how to remove a road block which is either built wrongly or you just need to re structure it.
Here is the way. Just tap on demolition button and select the road block. The selected road block will become gray. Once you select appropriate road block, you will see right sign to turn green. Thats it. Tap the green sign and the road block will be removed.