The hyphenated write-off is a noun meaning (1) a cancellation of an item from account books, or (2) a reduction of taxable income in recognition of an expense. In all other senses, write off is a two-word phrasal verb, with no hyphen. A write-off is something you write off.
The one-word writeoff is gaining ground in place of the hyphenated form, and it may gain wide acceptance someday. For now, though, publishers by and large prefer write-off.
Examples
Write-off
His wife, Barbara, was paid $42,600 by St. John Fisher College and took a small write-off for a human resources consulting business. [Wall Street Journal]
The third-biggest write-off came from scrapping armoured vehicles, which represented a loss of £431 million. [Scotsman]
While two of the units may be write-offs, the third unit only saw damage to its roof and exterior siding. [Calgary Herald]
Write off
The Dragonheart star, 57, said that some producers would make it available to actors and write off the cost as petty cash. [Mirror]
But those investors who want to write off the world’s biggest economy would do well to recall America’s astonishing capacity for reinvention and renewal. [Sydney Morning Herald]
Some taxpayers are able to write off most or all of their taxable income with personal deductions. [San Francisco Chronicle]