How to unassign a 1.14 Minecraft Villager's career in survival, no villager farm

The Village & Pillage Update (v1.14) now lets you control a Villager's Profession, with the use of job site blocks or "Workstations". These blocks can be "claimed" by Villagers, at a 1:1 ratio (one villager to one workstation).

If a Villager's workstation is destroyed, or they can no longer path find to that block (this can include moving beyond a certain radius of the block), they will lose their Profession, and can then be re-assigned a new one. Villagers must be at least one block away from a workstation, and be able to pathfind to the block to be assigned a new profession.

The exceptions to this are that Villagers cannot be Nitwits, or babies. Villagers that you have traded with prior to v1.14 can still be assigned a new Profession by workstations. Additionally, there is a bug in v1.14 pre-release 4 that can be abused:

Cured zombie villagers lose profession