1. Select the tree-tool
2. Start painting some trees
3. Hold down the shift key while painting trees
4. Erase stuff

Erasing an Arcology will still yield the population of an Arcology, but the Arcology will be gone along with it's pollution and crime.

Erasing half of a power-plant would cut it's pollution in half, but it would also cut it's power-output in half.

Michael Weburg used the Magic Eraser trick to win the ClubOpolis Population Contest. Taking a clue from erasing Arcologies, he erased a 2x3 area of a 3x3 building. He then overlapped the erased section of building with a new 3x3 building. The tiles left over in the 1x3 building still added up to the population in a 3x3 building. Continuting to do this for his entire city, he ended up with hundreds of 1x3 buildings each with the population of a 3x3 building.

If he had the patience and time, he could have reduced each 3x3 building to a 1x1 building thus giving him almost 6 million sims not counting Arcos. Throw in 140 Launch Arcos and he could have a potential population of nearly 15 million. His winning city was 11.2+ million (the contest limited cities to 140 Arcos).