Providing a completed W-9 upon request is required. That's why it's best to ask for the form or TIN before/when signing the lease or before making a payment --that gives the person time to withdraw from the transaction if they don't want to provide it.
I think KevinM is correct that if they don't provide the completed W-9 upon request or their TIN then you do withholdings. This form states the rate at 24%. In all the years we've farmed we've never had anyone refuse to provide it tho.
https://content.govdelivery.com/accounts/USIRS/bulletins/2153657 | 
Form for landlord to give SSN for 1099
