Another hitTest question (Flash MX)

I’m trying to make a maze game, and my problem is that i can’t make the ball stop where I want it to. The walls are thin, and if the ball has a high speed it just goes through them. I want the hitTest to detect if any part of the ball touches the wall, not just the center point. This is the code for the ball:

}onClipEvent (enterFrame) {
if (_root.vertical.hitTest(_x, _y, true)){
xspeed *= -0.2;
_x = posx;

if (_root.horizontal.hitTest(_x, _y, true)){
yspeed *= -0.2;
_y = posy;

Can anyone help me? Is my explanation understandable? Thanks in advance.