#include <amxmodx>
#include <hamsandwich>

new bool:change_name[33];

public plugin_init() {
RegisterHam(Ham_Spawn, "player", "player_spawn_post", 1);
}

public player_spawn_post(id) {
if(!change_name[id]) {
new user_name[33];
get_user_info(id, "name", user_name, charsmax(user_name)); //или get_user_name

if(!IsValidString(user_name)) {
set_user_info(id, "name", "New nickname");
}

change_name[id] = true;
}
}