You can do that in two ways, directly or indirectly.
The direct way is, there is a menu under the ‘Basic setting’ page which is ‘exclude user’. just put the id/name of the user in that box/input field and ‘save changes’. That will block that user from getting points and rewards.
Another way is, ‘suspend’ a user from the ‘log’ report. To do that, go to ‘log’ from the sub-menu of ‘Pointpress’, you will find the whole statistics on your all event. Find the specific event of a specific user. You will see three icons, click the ‘man icon’ that will pop up the ‘manage user’ box. Just click ‘suspend this user’ to suspend a user.