It is impossible to decline the pennant when Hasse gives it on day 13, but it is possible to keep it when he offers to buy it back for 10 credits on day 22. This is done by detaining Hasse when he refuses to leave without the pennant. If Hasse is detained, the pennant will remain in the booth for the rest of the game. The pennant itself will not grant any gameplay benefits, so choosing between keeping it and returning it to Hasse is purely a choice of conscience.
As with the family photo and the Son's Drawing, having the pennant on the wall during the inspection by the M.O.A. supervisor at the beginning of days 20 or 30 will lead to a fine or can possibly end the game (ending 11).