Search for the text in src/map/clif.c, comment or delete the relevant lines, and recompile.
// if player is autotrading
if (dstsd->state.autotrade == 1) {
char output[256];
safesnprintf(output,sizeof(output),"%s is in autotrade mode and cannot receive whispered messages.", dstsd->status.name);
clif_wis_message(fd, wisp_server_name, output, strlen(output) + 1);
return;
}