This has been discussed elsewhere at some length and the general consensus would appear to be that it is extremely difficult. I managed it by allowing people to use the programme for a fixed period and then they had to contact me for a password if they wished to carry on using it.
Unfortunately, because I linked the password to the actual machine that the user had installed the programme onto, it meant that people who had legitimate reason to want to use the programme on another machine ( new computer etc...) also had to contact me. This hasn't proven to be much of a problem. Bottom line is if you are prepared to be involved then you can. If not then it is pretty darned difficult.