Well, depending on how you marketed the initial installment, you could be the one at fault. The industry norm is that quality and security updates are free for the support life of the product (which is usually longer than you say it is). In some jurisdictions you might have no legal right to deny the updates, regardless of what your contract says about warranty.
If you want to save yourself the hassle, factor the cost of maintenance updates into the initial cost, and market upgrades as big new releases like Windows or AutoCAD do.
Which product is this, by the way?