I am too kind of bad with CSS, I don't think I wrote any of the CSS but a border radius should function (I think I added style properties to this? I don't really remember. lol)
You can turn the option off (search userTagKeepAt in the ACP), but any pre-existing messages will need to have it edited out. There is no easy method to accomplish this besides direct DB query, but you shouldn't do that unless you are comfortable.