In the WA_Globals.php file, there are a number of settings that effect how readable the captcha image is. The $WAGLOBAL_Captcha_Noise setting controls the color of the noise. Set it to white so it is the same color as the background to remove it:
$WAGLOBAL_Captcha_Noise = "ffffff" ;
or use a color that contrasts with the captcha text color, for example, red if the text is black:
$WAGLOBAL_Captcha_Noise = "ff0000" ;
$WAGLOBAL_Captcha_Text = "000000" ;
At this time, the Contact Form Solution pack does not have support for redrawing the captcha image. You can post this to the wish list to have it considered for a future update.