Each axes instance contains a lines attribute, which is a list of the data series in the plot, added in chronological order. To delete a particular data series, one must simply delete the appropriate element of the lines list and redraw if necessary.
The is illustrated in the following example from an interactive session:
>>> x = N.arange(10)
>>> fig = P.figure()
>>> ax = fig.add_subplot(111)
>>> ax.plot(x)
[<matplotlib.lines.Line2D instance at 0x427ce7ec>]
>>> ax.plot(x+10)
[<matplotlib.lines.Line2D instance at 0x427ce88c>]
>>> ax.plot(x+20)
[<matplotlib.lines.Line2D instance at 0x427ce9ac>]
>>> P.show()
>>> ax.lines
[<matplotlib.lines.Line2D instance at 0x427ce7ec>,
<matplotlib.lines.Line2D instance at 0x427ce88c>,
<matplotlib.lines.Line2D instance at 0x427ce9ac>]
>>> del ax.lines[1]
>>> P.show()
which will plot three lines, and then delete the second.