Perform the following to delete one or more trend lines from the Chart window:
From the Chart
toolbar, click 
NOTE: If you do not see this button, you need to display
the Chart toolbar.
Perform one or more of the following:
To delete a single trend line, click the trend line you want to delete so that it is selected, and then press the Delete key. The trend line disappears.
To delete multiple
trend lines, hold down the CTRL
key and then click each trend line you want to delete so that they are
selected.
Release the CTRL key. Press the
Delete key and the trend lines
disappear
TIP: You can also select multiple trend lines by clicking and
holding the mouse button while drawing a selection square over all of
the lines you want to select.