1 simple way you could add a text box which is invisble to the user with some code on it that counts the number of times it has been opened if it reaches a certain number than it runs an uninstall program or just displays an ordering form on load!

I.e

Has invisble text box or maybe just a string!
Onload of form it adds 1 to the string.
When it reaches say 30 everytime from then all froms are hidden except the registration one!

hope it helps