/ip firewall filter
add action=drop chain=forward dst-address-list=RoV src-address=192.168.50.0/24 comment="Blocking Garena RoV traffic using RouterOS"

/ip firewall mangle
add action=add-dst-to-address-list address-list=RoV address-list-timeout=4d chain=prerouting dst-port=20000 protocol=tcp comment="Detecting IP Addresses Garena RoV (Sv.TH)"