If demo users play but do not intend to pay for more its mean they dont find LFS worth paying there is nothing you can do there
If you going to limit them I afraid it would result in tons of new accounts the cons are there will be tons of junk data on servers and pool of the user names will be more and more limited with time as more and more names would be taken by the same demo users.
Also different ppl learn with different speed.
My PB on blackwood is still 1.37
a lot of ppl need more time than month to be able to compete with others and have any fun from racing cuz its no fun being last.
So cutting this time wouldn't do any good for the game IMO.
Also demo users still figure as active players which is shown as racers online you my consider its a cheating but for new ppl (Potential buyers) who visit lfs.net to check what kind of game it is, it would be first sign that game is not dying so its kinda help to make ppl decision if they want to invest their money in game with perspective.
And the last banning them from online after a month for example doesnt mean they stop playing you know CRACKS and stuff so basically doing anything would be a waste of time and effort.
Someone who doesn't want to buy a game and play as a demo user for years wont buy it just because his demo expires.