Once endless energy is available, dismantling things is not very hard. It would be trivial to, say, subject the mountain range to the energy equivalent of the entire solar flux received by the earth. The forces of ten thousand suns would literally melt the mountains into lava.
The larger issue with such technologies is that others have the technology too. Naturally, if Young Alice is the only mad scientist on the block with a tectonic scale heat ray at her disposal, she can get rid of mountains if she pleases, and no one will complain (at least not to her face). However, assuming others have this capability, restraining use of great force is far more challenging than the uncontrolled use of it.
So let's start by restraining Young Alice's heat gun with a safety interlock. It's only allowed to slag mountains if the matter has been put to a vote, and the plans have been on display for at least 3 months (and not in the corner of some government building basement either). Then a federal officer would come by, deactivate the interlock, and for a few joyful moments, Alice would be master of her own environment.
Of course this isn't the most interesting outcome in the world. Especially since she won't be the only one with a safety interlocked heat gun, and the government faculties devoted to the tactful removal of granite and limestone would be swamped with requests.
It's also not all that interesting because, frankly, there's no reason for everyone to have the power to slag a mountain if it's locked up all the time. More likely the heat gun will have configurable power. The maximum mountain slagging power would require a full federal process, while the power needed to, say, toast s'mores would be freely available without any interlocks. Naturally you would want a range inbetween. Perhaps eradicating an oak tree blocking your view of the mountains would only require HOA approval. That way you could enjoy your view before Alice gets her mountain-slagging pass and leaves you with a view that you wish was obscured by an oak tree!
As you get to smaller scales, we find more desire to create than destroy. Your neighbors may not accept you demolishing a tree, but they may let you express your inner artist by turning it into a topiary. With the limitless energy of a sun, it would not be surprising if we had such technology.
Let's make this a bit more compact. The limitless powers we are talking about are so magical, let's just make it a magic wand and be done with it. The magic wand has a bunch of safety interlocks, but once you unlock the right level, it lets you create a particular flavor of nanobot to do your bidding.
That's better! Now let's address the voting process. Nanomachines don't do voting like we tend to. They're too small and there'd be too much information to deal with. Instead, they do something known as "quorum sensing." It's a lot like voting, but it can be done in a more localized and continuous process. Yeast use it to decide whether the environment is a "go forth and multiply" environment, or a "hold onto your butts!" environment.
So there would be nano machines at the mountain quorum sensing what should be done to the mountain. There would also be a stream of activated nanobots between Young Alice and the mountain, conveying her desires. This process can be quite intelligent. If she wants to carve the mountain into Mt. Rushmore, and the only other opinion is from someone on the other side of the mountain, it may give her the ability to carve her side. If she wants to obliterate it, she'll have trouble achieving quorum with someone on the other side opposing her.
Of course this whole process can be a bit stressful. If you were a microscopic particle trying to stand between a woman trying to slag her husband's holey boxers using a magic wand, and her husband desperately trying to resist, you'd be stressed too. So the nanomachines are going to have to have some communal ability to recommend lower stress alternatives (such as accidentally "misplacing" them the next time they need to be washed).
So the biggest opposition Young Alice faces in her mountain moving exercise is the voice of those who think mountains look nice. As her frustration builds, she starts pushing on the nanomachines to do her will rather than the others. The nanomachines at the mountain give feedback: "She's stressing us out, you upstream nanomachines need to help us out!"
Fortunately, as a general rule, if you're focusing on one thing, you're not focusing on another. If Young Alice is focused on slagging mountains, she's not focusing quite so much on the state of her lovely garden. Thus she won't mind too much if instead we slag a little bit of the ground in her garden to form a tunnel, much like the kind a rabbit might dig. After all, it's only a small hole. Nobody will mind much. We might not even mind if the nanomachines construct the shape of a White Rabbit with pink eyes to run close by her.
Oh dear! Oh dear! I shall be too late!