Honestly, if you make enough levels (or even if you only made a few like I did), sooner or later you will end up in the unfortunate position of having one or more levels that you simply cannot backroute-proof as-is, instead requiring some form of either very radical redesign, or accepting and adopting the backroute as intended solution, or just plain abandon the level and design something else.
This. My level "Crossing the Nile" is still un-fixable because any backroute possible requires fewer skills than my overly complicated intended solution.
@Crane:
Regarding knowledge of tricks, I hate to exploit your despair for shameless self-promotion, but the first two ranks of my pack Lemmings World Tour are precisely about that: Teaching the player a bunch of obscure tricks.
In fact, I was frustrated about the same thing as you: People using obscure tricks and skill combinations that you either knew about or not, and which, in the latter case, you were more likely to find out about by reading about them rather than by trial and error.
Have you tried fixing your level with pickup skills? If the blocker isn't on the skill panel yet while the lemming is diggging, or you don't have the digger yet when you need to block, you can prevent the player from applying this trick simply due to not having both skills at their disposal simultaneously.
This doesn't always work, of course, but it's usually my first go-to attempt when other factors, such as raising the save requirement to prevent blocker usage etc., don't suffice to eliminate a backroute.