Define “purify.” To purify something means to remove anything unwanted or deemed as negative. So if you are planning on using a room for demonic magick, for example, to purify the room might mean removing any angelic influence before consecrating it to the demonic.
If you mean to simply clear it of unwanted energies, then there are a variety of methods to do so.
The simplest method I know of is EA’s banishing incantation from Kingdoms of Flame, or the angelic banishing Micah channeled here:
They both work to clear an area of all energies whatsoever.