It does not reduce the reward of fasting. A haraam action that is done after the fast ends does not reduce its reward, but that is included in the words of Allaah (interpretation of the meaning):
“and eat and drink but waste not by extravagance, certainly He (Allaah) likes not Al‑Musrifoon (those who waste by extravagance)”
Extravagance itself is forbidden, and moderation is essential. If they have more than they need, they should give it in charity, for that is better. End quote.
Shaykh Muhammad ibn ‘Uthaymeen (may Allaah have mercy on him).