Google Sheets: Prevent the unchecking of a checkbox after it has been checked

Solution:

You can take advantage of the Class Protection to protect the cell if the checkbox is checked; value becomes TRUE.

function onEdit(e) {

  const row = e.range.getRow();
  const col = e.range.getColumn();
  const as = e.source.getActiveSheet();

  if(as.getName() == "Sheet1" && col == 2 && row > 1 && e.range.getValue()===true) {  
     
   const protection = as.getRange(row,col).protect();
    
  // Ensure the current user is an editor before removing others. Otherwise, if the user's edit
  // permission comes from a group, the script throws an exception upon removing the group.
   const me = Session.getEffectiveUser();
   protection.addEditor(me);
   protection.removeEditors(protection.getEditors());
   if (protection.canDomainEdit()) {
  protection.setDomainEdit(false);
  }
    
  }
}

• Feel free to modify "Sheet1" and col == 2 (2 corresponds to column B).
• To enable this code snippet, copy & paste it to the script editor (Tools => Script Editor) and click on save.

Example:

Protect the cell or the sheet after the checkbox has been checked. This could be done manually i.e. right click the cell then select protect to open the sidebar to set the protection settings, or automatically by using Google Apps Script.

Resources

• https://developers.google.com/apps-script/guides/sheets