Tricky Turn

[GTSamIAm]

This was against an unknown. What is my optimal turn line here? I bet to prevent giving him a free card to draw to a possible flush. But when he raises, at this limit, do you think he is semi-bluffing or bluffing? I'm getting about 15-2 on a call down here.

Party Poker 0.5/1 Hold'em (9 handed) internettexasholdem.com

Preflop: Hero is MP3 with K[img]/images/graemlins/diamond.gif[/img], A[img]/images/graemlins/heart.gif[/img].
UTG calls, UTG+1 calls, <font color="#666666">2 folds</font>, <font color="#CC3333">Hero raises</font>, CO calls, <font color="#666666">3 folds</font>, UTG calls, UTG+1 calls.

Flop: (9.50 SB) 4[img]/images/graemlins/spade.gif[/img], 4[img]/images/graemlins/club.gif[/img], K[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(4 players)</font>
UTG checks, UTG+1 checks, <font color="#CC3333">Hero bets</font>, <font color="#CC3333">CO raises</font>, UTG folds, UTG+1 folds, <font color="#CC3333">Hero 3-bets</font>, CO calls.

Turn: (7.75 BB) 3[img]/images/graemlins/club.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">CO raises</font>, Hero folds.

Final Pot: 10.75 BB