How do I prevent new villager trades from spawning?

One of you said that the max career level is 4, so I used this command

/summon Villager ~ ~1 ~ {Profession:0,Career:2,CareerLevel:4,Offers:{Recipes:[{rewardExp:0b,maxUses:2147483647,uses:0,buy:{id:double_plant,Count:8,tag:{display:{Name:"Coin"}}},sell:{id:wooden_sword,Count:1,tag:{Unbreakable:1,display:{Name:"Sturdy Stick"}}}},{rewardExp:0b,maxUses:2147483647,uses:0,buy:{id:double_plant,Count:12,tag:{display:{Name:"Coin"}}},sell:{id:golden_sword,Count:1,tag:{AttributeModifiers:[{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:-.10,Operation:1,UUIDLeast:112181,UUIDMost:524106}],Unbreakable:1,ench:[{id:16,lvl:1}]}}},{rewardExp:0b,maxUses:2147483647,uses:0,buy:{id:double_plant,Count:2,tag:{display:{Name:"Coin"}}},sell:{id:leather,Count:1}}]},Attributes:[{Name:generic.movementSpeed,Base:0}],PersistenceRequired:1,ActiveEffects:[{Id:10,Amplifier:50,Duration:133333333333320,ShowParticles:0b},{Id:21,Amplifier:60,Duration:1333333333333333200}]} 

and set the career level at 4, this spawns a villager with a career level of 4 that will trade sunflowers named Coin for certain items. Now I tested this multiple times to make sure I was correct, and I was, setting the career level at 4 will prevent new trades from appearing. I am not sure if this works in 1.9, 1.10 or 1.11 because I still play in 1.8.9.

I don't know much about custom villager trading, and a cursory glance over the NBT structure of villagers didn't reveal anything new to me.

As for your second point, there's a perfect technical block that should work for the newer versions: The Barrier.

Not only is it unobtainable without creative/commands, but it's icon is this:

It's itemID is barrier, so you can try

/summon Villager ~ ~ ~ {
  Offers:{Recipes:[
    {maxUses:1337,buy:{id:diamond},sell:{id:emerald}},
    {maxUses:1,buy:{id:barrier,tag:{display:{Name:No More Trades}}},sell:{id:barrier,tag:{display:{Name:No More Trades}}}}
  ]}}

Which sells renamed Barriers for renamed Barriers. You might need to use minecraft:barrier instead of just barrier, I'm not sure about that.

I'm almost one hundred percent certain that the other comment won't work. However, what will work is this:

/summon Villager ~ ~ ~ {CustomName:Billy,Profession:4,CareerLevel:6,Offers:{Recipes:[{maxUses:99999,buy:{id:"minecraft:emerald",Damage:0,Count:10},buyB:{id:"minecraft:book",Damage:0,Count:1},sell:{id:"minecraft:dye",Damage:5,Count:20}}]}}

When this command is triggered, it will summon a villager with the name Billy, who wears a white apron. He will trade you 20 purple dyes for 10 emeralds and 1 book. An odd trade to be sure, but it works.

I know you've probably either given up or found a work around by now, but this is for those who stumble across this question/answer later.