Comment Mikrotik RB750 (Score 0) 520
I do this, using a Mikrotik RB750 router - about $40 anywhere online. Even has virtual OS/router support. Using the firewall, block everything & open the ports you want (80, 443, 25, 110, 587, etc.). You could even redirect their DNS to OpenDNS or something else if you don't want them doing naughty things on your connection.