Adding ACL to filter traffic on LTM for incoming Traffic?

The proper solution is to use AFM otherwise yes you can use iRules though I would really look into AFM.

You can use packet-filters, AFM or iRules. For just a few IP addresses, I would say use iRule. If you are looking for something along the lines of a stateful filtering, AFM is a great solution with packet-filters falling between the 2 solutions.

Your iRule looks good. Use the log statement to make sure the right IP address is being seen by the F5. Sometimes the original IP address may be masked by a proxy of some kind.

when CLIENT_ACCEPTED { 
if { not ( [class match [IP::client_addr] equals ALLOWEDIPS] ) } { 
log local0. "[IP::client_addr]"
reject 
} 
}