This is a bug, as reported on the official Minecraft Bug Tracker.
It was reported as fixed in 14w02a (9/1/14) but reopened yesterdat (8/2/14), so no new date for fixing has been given. However, given that the bug was fixed once before, it is highly likely it will be fixed again soon.
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.
Best Answer
If you mean just the display name, then a custom name will replace it:
If you mean to prevent careers from affecting trades (since trading will refresh trades based on their career), then you need to set their "Career" to a non-0 value that also corresponds to their "Profession" (a value of 1 covers all professions), as well as set their "CareerLevel" to a value higher than the 'maximum' level for that career (which varies).
For example, the following villager will never create new trades.