Smooth g Delete data points

Deletes all data points within a rectangle you just drew. Handy for deleting single spikes from a plot. 

g Discard (shift-Del)
Throws away the data points. The total number of data points in your plot will decrease as a result.

g Interpolate (ctrl-Del)
Does not throw away the unwanted data points, but puts them "into place". Spectrum Viewer will draw a line between the surrounding valid data points and place the offending data points on that line. The X value of the data point will be left alone, the Y value is calculated so that the data point is placed on the line.

For data sets with a lot of spikes it is better to choose the delete large spikes command. See also this tutorial.