How far do crimson/corruption biomes spread? [duplicate]

The Clentaminator, along with Green Solution can be used to purify it. However, these are sold by the Steampunker, who only appears in hardmode (the condition for selling individual solutions varies; the Green Solution only appears when no other solution is being sold).

You have other options. Firstly, you can build upward. The corruption rating of a home counts all corrupted tiles in a 45-tile radius. Moving upwards dramatically reduces the odds of that being an issue. If you go up high enough you will be outside of the Biome. There's no need to worry about making your homes accessible from the land; if someone moves in you can just save + quit and reload and they'll be in the home.

Secondly, activating hard mode of course spawns Hallowed and Corruption (or Crimson, in your case). Hallowed does not prevent homes from being suitable, fwiw, so if you can build that way you will also be safe.