Pretty much any of them can be damaged and presumably destroyed, though it's questionable how effective it really is with some of the older ones. They've thrown everything in the book at them over the years and it seldom does more than slow them down.
I know it's comics, but Kamen Rider Spirits seriously goes apeshit with the belt destruction, and everybody's still going strong.
[HIDE]V3's gets severely damaged and even after Riderman patches it up, he blows it out using the 26th Secret when he's got no choice. And Kazami is so badass he's still up and running after that! Wielding Riderman's machine gun arm (it's a long story) since he can't turn into V3 anymore. Muraeda's as much of a diehard trad fan as I am though, so I'm sure he'll put the toys back in the box by the end, and has planned out some way to fix it.[/HIDE]
Also, in the recently-concluded flashback storyline of New Kamen Rider Spirits
[HIDE]Soon after they first met, Rider 2 gets shot right through his belt, but he lives on, and Rider 1 fixes him up. And this apparently is why he's later got the TRC logo on the opening panel thingies since.[/HIDE]