It seems nobody mentioned already, but locking people out of the system when your support expires when you have sold them a V5 license is surely against the terms of the V5 license you purchased on behalf of your customer for SambaPOS.
As others have said, this is a very bad idea. And considering you say yourself you are a new to this, you surely are not in a position to be locking down your customers. This idea goes against everything SambaPOS is designed for, it’s not meant to be resold and locked down by a reseller to the point that the end customer cannot use it without paying that reseller for support.
Also, I had a feeling I heard the same mentioned before, and I checked back through your post activity. You asked the same question before in 2015 and were told then about not doing this. So PLEASE do not do this, there are many resellers including myself who provide a quality service to our customers and don’t lock them in like this (my customers pay me for support based on the service I provide, I don’t need to “lock them in” like this). Doing what you intend will give customers a bad experience and indirectly then they will link this with the name of SambaPOS as a whole, and I think you should consider strongly about that. You come on the forum here and get free support from people, yet you want to take and not give back. You want to stop your own customers getting the help you get. This is not a business model based on providing a quality service to your paying customers.