The affiliate accounts can't be deleted. However, you can deactivate affiliate's account and prevent the given affiliate access to your Affiliate program if necessary.
To do so, open any affiliate profile (by clicking on the row with the affiliate's name).
Look for a field Status and choose the option Disabled. Once you do so, the affiliate won't be able to log in to your Affiliate program again unless you activate his account.
You can also decide if you only block the affiliate's access to the platform (Status) or if you choose to block also his traffic (Traffic Status). These are two independent options. If you wish to block all the clicks to not pay for affiliate's generated commissions, you have to also block their traffic by changing the Traffic status field to Disabled.