There is a rule from the old Tribunal that is still used by Riot staff to judge intentional feeding cases. Some veterans of the Tribunal might remember it as the AP Vayne Rule or the AP Garen Rule. Basically, Garen and Vayne have no AP scalings, so if someone builds AP on them, then it's bannable.
- To build AP, the champion in question must have at least one ability that scales based on their AP.
- To build AD, the champion in question must have at least one ability that scales based on their AD, OR have an attack damage or attack speed steroid in their kit.
- To build Mana, the champion in question must use Mana as a resource, or the item in question must have the Spellblade passive.
- To build Attack Speed, Lifesteal, or Critical Strike, the champion in question must have an attack damage or attack speed steroid in their kit or have an ability that increases damage based on the stat in question.
- To build Spell Vamp, the champion in question must have a direct damage ability in their kit.
As far as non-meta picks, the AP Garen Rule has a corollary, the Support Yi Rule.
T) All champions are valid top lane picks.
M) All champions are valid mid lane picks.
B) Any champion who benefits from attack speed under the AP Garen Rule or named Mordekaiser is a valid bot pick.
S) Any champion with an ability with a movement CC on it or a heal or shield that can affect allies is a valid support pick.
J) Any champion that can tank and kill one of the clear camps with a leash at level 1 and immediately go solo a buff camp at level 2 without going to base to heal in between is a viable jungle pick. (Gromp and Krugs are clear camps, red and blue are buff camps, and wolves and raptors are vision camps)
The ruling on viable jungle champions is actually forcing many to take Smite as well. There's a couple who don't 
but the requirement of "kill camp with a leash then go solo the buff at level 2" forces almost all champions who wish to jungle to take Smite.
So, your Leblanc jungle without Smite is bannable. Leblanc cannot tank and kill either the frog or golem and then go kill blue or red without assistance or recalling, especially without Smite. However, something off-meta that still falls in these general rules is not bannable. An example would be Darius or Riven jungle. Darius and Riven are subpar junglers, but both can tank and kill a clear camp and immediately go solo a buff camp.
These rules are how I and all the other legitimate Tribunal judges worked cases when Tribunal was up. Now that Riot judges cases, Riot's judges use similar criteria. Not every judge used the same criteria, but they all followed a model like this to allow for non-meta play but ban trolls anyway.
