Aiyooo I know lah, at that age memang hard to stop! One of my mum friends tried putting bitter stuff on the fingers (like the anti-thumb sucking cream), tapi her son just licked it off hahaha. What worked better for us is finding out WHY dia hisap, sometimes it's comfort, sometimes boredom or teething, so try pairing it with something else macam soft toy or blanket during bedtime routine, and praise like crazy when dia tidur without doing it. Be patient jugak, some kids naturally stop around 3-4 years, jangan stress too much because the more you force it the more they hold on to the habit!