You can call it what you want. If the pilot doesn't want to play there isn't anything you can do about it in class G.
And forcing what you think is best on the pilot may lead to the pilot withdrawing from taking a service from you or for that matter putting a plan in ever again.
There needs to be consistency across the whole of the FIR's, if they find out that the unit before yours is happy to close the plan that's what they are going to start doing.
By applying your best practise and what you think is right just confuses the hell out of the pilots, Stick to the rules. If there is nothing in the book which says you can't and the PIC request's it do it. The pilot won't know or care what's in your MATZ part 1 or MATZ part 2. If country wide its decided that its not allowed to close the plan in the air get the rule book changed and document it so every pilot is using the same rule book with every ATS.
You can't refuse a "cancel my flight plan" so to me its a bit of a bun fight about nothing and who is really in charge of the way a flight is operated.