The best way is to assign a unique person ID to the person, and set the flag with that person ID. So, if you gave the person an ID of 0027, using "setflag 0x27" will make him disappear permanently.
Ah, it worked. Just for the record, it was a script tile. But the person that the player talks to was in a "looking down" state. Changed it to "no movement" and it worked. Thanks!