Another of my sites has been hacked and the home page replaced. Much less damage this time and it was easier to repair, remove and make the site secure again. (The cause was an old Drupal installation which I tried and abandoned but did not delete. It had some bug in it that gave people more access rights to the site than it should.)
Unfortunately for the numpty children doing this, they don’t realise that putting graffiti on walls does not endear people to their cause. Assuming it really is them and not someone else trying to discredit them.