If they don't want to buy it you can't force them to but to delete them because they don't is over the top also you will lose all their posts and google index will cause errors with all the missing posts
By removing them, you're loosing for ever the possibility to have them at any time in the future as paid members. Don't forget that 90+% of the user, first want to test your site and then decide to pay for access.
By removing their accounts (which means new registration), simple you'll get a sh*t from them. Nobody is willing to register again especially if he has to pay.
My opinion, similar to ForseHSS, is to redirect them after login, in a page where they can buy a subscription. But don't delete them. If I remember well there is such a plugin here.
Is there a way to delete users who have never ever bought a paid subscription?
There is, IF you maintained a record of users who did purchased and who did not.. (appropriate usergroups or paid subscription list)
Which is doable via Vbulletin.. hope it makes sense ?
Instead of deleting them, why not just put them in another Usergroup that has very limited permissions. Something like the Banned Usergroup, but not exactly.