Moshe,

There are actually a couple of ways of doing this. If you don't want airports ever added at all, you can do as JS suggests, and select the option for "Do not add other airports..."

You can also mark the airport as unlandable instead of deleting it. This is what I would recommend. If you are using SeeYou simply edit the airport and change its style to "Unknown". This will make the airport no longer show up in your glide computer's list of landable places. Soaringdata.info will preserve that style whenever you process the file.

If you use a text editor to maintain the file, you will find the style field right after the airport elevation. Change the value (usually 2, 4, or 5 for airports) to a 0 instead.

L.