A firm, brief letter is surely Darcy's best option to secure Wickham's quitting the neighborhood. (Me wonders if Darcy hinted he would make known Wickham's address to various local tradesmen Wickham may've owed money too.) That might add a threat of authority;) Anyway, after Darcy's letter, Wickham realized there was nothing for him in Ramsgate and left. Darcy often appears to rely on letters for communication.