Have you worked with US Forest Service's open source software Fusion? It processes LiDAR data in DOS environment.
This software converts ASC files to ".las" or ".lda" (see pages 6 and 23)
I would try the following program command: ASCIIIMPORT (see the manual's page 23).
Here is how you can do it.
Install Fusion (place it at top hierarchy, right under c:).
Open Fusion's Main Screen.
Click "Tools", choose "Data conversion", and then: "Import Generic ASCII LIDAR data...".
Browse you Ascii ".txt" file. Save it as: ".importparam" extension.
Install Notepad++. Save notepad++ ".txt" file as ".bat".
Let's suppose now:
a) Fusion is installed at the following directory: c:\Fusion;
b) both the ASCII ".txt" and ".importparam" files are stored in: c:\LIDAR;
c) the name of the files above are: "project.txt" and "project.importparam";
d) the output (".las") filename will be "project.las".
This is the syntax of Fusion's ASCIIIMPORT program command:
ASCIIImport [switches] ParamFile InputFile [OutputFile]
Using examples above, write the following code in Notepad++:
c:\Fusion\Asciiimport las/ c:\LIDAR\project.importparam c:\LIDAR\project.txt c:\LIDAR\project.las
Save the notepad++ ".bat" file, before run it. Press F5 to execute it.
Once you have your files in ".las" format, you can use the program command named GroundFilter (manual's page 88). It will apply the algorithm of Kraus and Pfeifer (1998) (see manual's page 89) that surely will remove your ground returns outliers.
Here is one answer (with code example) which shows how to run/execute GroundFilter.
EDIT: added code example of how to run ASCIIIMPORT and a link with code example of how to execute GroundFilter.