Afaik not possible if you want to distribute the MT2 file. Only way I guess is to distribute the music as an mp3 or similar. Or that way at least it won't be easy to extract a particular sample.
If you want to distribute the MT2 then I guess the only way to "lock" the sample is to say in the infobox that you don't want others to take this sample without your written permission. That's not actually going to stop anyone from taking the sample if they want to though...
Another way is to mix two lines into one big sample. The size of the module might be bigger, but you'll decrease the risk of someone stealing your samples..