Security Advice

[xyn]

Hi Guys,
I'm looking for a way to secure a license checking tool into my products
basically when a client purchases a product if they need to pay per month
i wanted their product to always check with my database on my site that
they have a valid license key.

If their key was activated they would not be able to re-use this product
this is so only one product is used on one key. how ever I was thinking
to include a "in-between" into my site and when the products config page
is opened (usually included in every page) it sends their license ID to my
site via a $License_ID variable.

Obviously they can remove the "include" and off they go with a product
they dont need to pay for. Is there anthing i can do to secure my method
or any better methods to use?

-Ash