Griefprevention - Claims getting griefed, need quick help

Hey friends, I need quick help on this.

I am using griefprevention version 7.8 on my server. Players on server claim their area by stick/shovel but after sometimes somebody comes and raids it. They don't use /trust on other people to allow them to build on their claims but still after sometimes somebody's plot is griefed + It is always claimed by the griefer.

My friend ahmed was griefed 3 times this way and all the time his builds were claimed by somebody else and griefed.