The solutions could be quite simple:
-Deny placing wards too close from each other. In a moderate range, this would help people from spamming them too close from each other.
*As a secondary effect, this would help 2 teammates when they ward almost at the same time, denying one of the players from adding the ward in the wished position, saving potential vision.
*Yeah, that might affect strategy in the battlefield referring to places like ramps, but you, Riot guys, are awesome enough to program it properly.
-Add cooldown on ward placing. This idea might be bad in the battlefield actually. But like 3 seconds or so might be strategically enough to fit in battlefields.
*Thinking about it, I don't like the idea. But It's an idea, you can evaluate it
-Could these be good ideas?