I'm not a TEFL teacher (yet), but it seems to me that you could use pictures to convey the meaning of 'inconsistent'.
Inconsistent can mean a few things, but most of the time, its meaning is concerned with performance. Someone/thing is consistent if they/it produce predictable performances and results. If they are unpredictable, they are inconsistent (because you never know what you're going to get).
You could illustrate this with pictures of graphs (something like performance = x axis, time = y axis). One would be a fairly straight line over time, the other would be up and down like a yo-yo.
If you can make this in some way engaging for your students it would help. For example, do they have a favourite football team/player? Use them to illustrate it (e.g., "last season, Cristiano Ronaldo was consistent. He played well every week and the newspapers praised him. This season, though, he's playing well some weeks but not others, and the newspapers are saying some bad things about him. He is inconsistent.")
Hope that helps. One good thing is that as you're teaching Spanish speakers the words should be partly familiar to them, given the common origins of the Spanish and English languages.