If it is indeed list bullet items you can try putting !important in the style to make sure that there is no list style applied.
.ComboBoxStyle .ajax__combobox_itemlist
{
border: 1px solid YellowGreen;
font-size: medium;
font-family: Courier New;
padding-left: 0px;
list-style-type: none !important; /* Add the important here */
}
I can't reproduce your issue but you must have some sort of css style being applied to ul or li tags that is causing this. I know you mentioned you unchecked all the styling on the control in the browser but there must be something you missed.
Hopefully this works.