I provide a "Free" version of dSS to all of my clients, that periodically promotes plan's, software, etc..whatever I want to set the URL link's to.
The free version is very limited but provides me with ton's of good information, and the ability to market my business further...Can't really be free marketing.
I never have a need to uninstall this id, because well...I don't really offer a whole lot with it, and it doesn't cost me a penny to let someone have it for free. In the end it only increases my chances of a returning customer.
I track all paid for plans so that I know when they need to be shut off, terminated, etc. This is a big bread n butter winner for dSS because anytime someone goes outside the boundaries of the plan's time, you can remind them via a URL. I would advise against using any scare tactic's, because it's kind of un-ethical in my opinion. Just be informative, and people will always find more reasons to use your services, rather than leave them.
Ultimately, if you want to bill on a per PC license, you will need to create a per PC Client ID, and or track each Client ID's $ of licenses, and what computer's they are assigned to. I really don't recommend ever leaving your software available for download online, because any half-wit can spend enough time to brute force crack your password installer. Always CYA, by requiring customer's to agree to a contract of some kind setting their limitations of use, ownership, etc., before you even sell the software to them. Stick to these basic's, and you'll go a lot farther, because when you catch someone using your stuff illegally, you will have the right to go after them.