A word meaning to make a task so difficult that it is practically impossible

Three words come to mind

Repress = to control what people do, especially by using force

Cambridge Dictionary

Inhibit = to take an action that makes something less likely to happen, or that discourages someone from doing something:

Cambridge Dictionary

Stifle = to prevent something from happening, being expressed, or continuing:

Cambridge Dictionary

Of these, I suggest stifle because it seems the most general of the three, without the specific overtones of force in repress, while including or implying the taking of actions as in inhibit. But the choice may be best informed by context.

The stifling of voting might then be done by encumbering the voting process.

Encumber = to weigh someone or something down, or to make it difficult for someone to do something:

Cambridge Dictionary

"concealed prohibition", seems like a good fit.

e.g. "Abortion is legal throughout this country, even though accessbility varies from state to state. In some states, requirements are so severe and accessbility so scanty that you might as well call it concealed prohibition."