Thanks Paolo for the explanation. Actually the Google autocompleter was disable but still there was no track of that specific string in any en_US.po or it_IT.po files. So, since one of the possible options I could think of was just to eliminate that particular placeholder rather than editing the text, I managed to achieve it with CSS with this code.
#geodir_post_address_row input::-webkit-input-placeholder {
color: transparent!important;
}
#geodir_post_address_row input:-moz-placeholder { /* Firefox 18- */
color: transparent!important;
}
#geodir_post_address_row input::-moz-placeholder { /* Firefox 19+ */
color: transparent!important;
}
#geodir_post_address_row input:-ms-input-placeholder {
color: transparent!important;
}