It's certainly true that finding unique (and not-so-unique) ways to kill of immigrants is slightly more imaginative than simply lowering your population cap (which doesn't really work anyway, fyi.). However, I find the act of murder itself an unimaginative way to solve a population problem, no matter how imaginative you are at acting out the murder itself.
There are other ways to get rid of excess dwarves that do not involve killing them.
1. Embark on a harder area.
Simple really, if you find yourself with a lot of dwarves you DON'T want, I suggest you start in a biome where it's difficult to keep your dwarves alive. Nanoforts in terrifying areas (with mythical creatures), Zombie Giant Eagles, Dark Goblin fortresses, Rivers and Oceans.
2. Play a challenge fort.
Subsist entirely off of hunting or fishing. Survive off of animal husbandry. Don't use traps. Building aboveground. All of these methods of playing will result in dwarven deaths, which will naturally limit the number of dwarves that come to your outpost (this normally happens once you pass 20 dwarves in a fortress. Before you have 20 you will get incessant immigration waves, especially if one of your founders dies.)
3. Transform them into something else/ make them hostile.
While it's technically cheating, DF companion can be used to make immigrants hostile... or even change them into goblins/troglodytes/Giants/dragons. Honestly it can make them even more interesting in their own right.
4. Play in a world without a dwarven civ.
The Last Dwarves in the World5. Never kill a noble, and have a sheiff/forterss guard and jail.
Eventually you will fail to fulfill a mandate and somebody dies, hey it's even technically playing the 'right' way. whuddya know?
6. Don't export a lot of goods. (To the dwarven caravan)
If you don't export a lot, and don't make a lot, you don't get many immigrants. Many of the people who complain about popcaps are also buying out entire caravans every year. Think about it, if you were desperate for work and young, wouldn't a new and highly productive city that had 'high unemployment' be attractive to you?
7. Plenty more.
There are PLENTY of other ways to limit immigration that doesn't resort to murder or genocide. It's simply a fact that most players don't want to deal with it.
Closing statement: Killing off 'excess' dwarves is not creative, no matter how creatively you kill them. The act of murder itself is pretty much on par with playing with a popcap (while quite obviously less viscerally satisfying) and should you want immigration at a later point, murdering citizens is actually detrimental to play. Thus, I prefer population caps over the obvious alternative.
P.S. In my main fort for this version, I had deliberately (I state deliberately, because a number of my dwarves died without my consent) killed only two dwarves. The first was a baron who like adamantine. The second was his consort.