I think; just ignore him at this point (after two firm refusals)
I think explaining further even online will be perceived as encouragement or misperceived as interest. The sex issue really complicates it because he wants sex and is trying to "fix" the situation by re-re-re-asking your further consent to give up some more sex. In the future be more certain he accepts your desire for a one nighter only or pick someone you would like to see again. In both those possible cases, please meet in a public place and be more cautious with whom you share your home address. He may continue to try for awhile but if you are inconsistent he will interpret this as a go signal for "more asking/insisting/demanding". No means no and yes means yes so please decide and stay decided. I wobble with indecision myself... especially about "dating and sex" or "dating vs sex" (maybe) I think I mean both of them.

In conclusion: Be safe.