Fibrous taint is the primary means of taint spread, appearing as a purplish discoloration or network of threads over blocks. Any block infected with it marks not only that block, but its entire column from earth to sky, as the Tainted Land biome.
Fibrous taint cannot spread to water blocks or onto blocks covered by water. This means that oceans and rivers will stop the spread of taint. However, since there may be caverns below the water, taint may still spread underground.
It is possible to destroy fibrous taint with tools, but it takes a while. An easier way is to pour a bucket of water on an infested block and then quickly pick up the water again.