can anyone tell me how to block external IP at RB750?
Add rule in IP → Firewall: deny input src address.
Src address is the ip you want to drop from. In there you have some more options like what kind of traffic you want from this ip to be droped.
Are you wanting to drop it to the router or through the router?
Input chain for to the router and forward chain through the router.
Can you please give a sample to drop certain traffic through the router?