Keep at it, clicking the teeny tiny box on the lower left and then click the x in the upper right. Eventually, it will go away...and in its place will be a similar annoying notice on how to save 50% and get paid faster by your customers. Or follow this link to apply for a PPP loan (even if we might have been successful going through our own bank previously) Unfortunately for us there is no true "do not show this notice again" checkbox we can use.
Oh, wait, you are using Tsheets, though you were talking about the annoying ad each time we log in. And to make it worse the Tsheet ad appears in company files where I am not even using payroll.