"The plane cannot be dispelled, but a creature on the plane can destroy it by using limited wish, mage's disjunction, miracle, or wish and making a successful dispel check."
I'm assuming you already knew that you can't dispel it with a normal dispel and were talking about the dispel check involed with casting a limited wish, mage's disjunction, miracle, or wish to destroy it. I'm just putting that little quote up there for those reading this who don't know.
As for making the dispel check more dificult, you can cast Create Greater Demiplane to add the impeded magic trait to your plane and choose limited wish, mage's disjunction, miracle, and wish as the spells to be impeded. This will make it a little more dificult, requiring a concentration check of DC 20 + the level of the spell they're casting in order to destroy it.
If they still manage to cast the spell, they have to succeed on the dispel check and since dispel magic say's
"You can also use a targeted dispel to specifically end one spell affecting the target or one spell affecting an area (such as a wall of fire). You must name the specific spell effect to be targeted in this way. If your caster level check is equal to or higher than the DC of that spell, it ends. No other spells or effects on the target are dispelled if your check is not high enough to end the targeted effect."
Pretty much anything you can do to improve the DC of whatever demiplane spell you cast to create the demiplane in the first place will make it harder to dispel.