Hi Michael,
The Lines that you want to remove are MajorGridLines and MinorGridLines of the YAxis. They are visible by default when Office 2007 Skin is used. To be able to remove them you can set their Color to Transparent like this:
Greetings,
Evgenia
the Telerik team
Consider using RadControls for ASP.NET AJAX (built on top of the ASP.NET AJAX framework) as a replacement for the Telerik ASP.NET Classic controls, See the product support lifecycle here.