I found the free command line programs LAStools on the Internet. It is quite useful to manipulate LAS files and it comes with the las2las executable. I use it to remove the LiDAR points that I do not want and retain the points that I want.
To see how to use this las2las executable, type in the following in a Windows Command Prompt as shown below:
C:\> las2las -h
To remove all points except the LiDAR ground points from a LAS file, you can type in the following in the Windows Command Prompt as shown below.
C:\> las2las -i input.las -o output.las -keep_class 2
where the value 2 is the standar LiDAR classification for ground points.
Once I have filtered out the unwanted points, I then run GeoMedia Grid's Import LAS File to Feature Class command, which will run a little faster - because there would be less points to convert than before.