I use 512 cable with few users one of them cut my connection every time i log in the lan how stop this asshole

i use outpost firewall which block him from duplicating ip of the router i tried changing my mac address and ip it work for 1 min then cut again is there anything to stop him,i also block him with switch snifferI use 512 cable with few users one of them cut my connection every time i log in the lan how stop this asshole
Change your password.