Generally there is no by-law that would be needed to enforce this, and the EC is within their rights to stop for sale/for lease signs being erected.
There are two reasons that spring to mind even if you have only the model by-laws.
Firstly, the signs would be affixed to common property, either screwed to the wall or by posts into the ground. That is of course a no-no when dealing with common property as damage inevitably occurs.
Secondly, a for sale/for lease sign, even if it is entirely on the lot owners property (which I doubt) is highly visible from the street and is not in keeping with the strata scheme.
If all/majority of the owners in your scheme seriously want to add signs all over the building then you would want to pass a by-law and it should specify the location, suggested size, maximum size, method of fixing, who is responsible to make good damage to common property and length of time it could be placed before being removed.