In order to deactivate a participant, click on the Program Management widget as shown below:

Then click on Deactivate Participants:

Tick the box next to the participant(s) you wish to deactivate. You can select multiple participants if you wish:

You will then be requested to select a reason for why you wish to deactivate the participant(s) from a drop down menu as shown below. This reason will be listed nex to their name in the list of deactivated participants. Once you have selected a reason, click on De-activate Participants.