I assume that recognizing /recognition of and praise for the slightest improvement by the disruptive child is the best way. No-one doesn’t like encouragement and praise, and that (encouragement and recognition) will label them as a good student. This belief can make them keep /maintain a good impression in front of others.

It depends on the disruption. If you have a child who cannot focus, then finding the reason behind that will help. Are they full of energy? They may need a way to move their bodies as they learn. Kinesthetic movements and learning exercises may help. Do they have some physical reason for not focusing? Sometimes if a child has vision problems, for example, they may begin to disrupt because they cannot see the materials presented. These are just a few that come to mind. Praise is definitely a good way to help the child, but finding the cause for disruption will help in the long term.