If you think about it, the reason why we are able to kick is because of how our leg bones (in addition to the rest of our skeletal and muscular systems are set up) are designed.
A biped, humanoid digitigrade race on the other hand, might have trouble because of how their skeletal structure is set up.
We happened to be a plantigrade species which means our digits, or toes, are flat on the ground, while a digitigrade species walks on its digits, like a cat or dog does on it back legs.
In my research, I found mention of this: the GURPS Traveller books talking about the Vargr who are a biped, digitigrade species of humanoid wolves. It said that Vargr weren't able to kick because of their digitigrade leg and feet structures.
That digitigrade wolf was able to kick that human. While it is possible that the webcomic creator may have used creative licensing (and he has been known to do that from time to time actually) and just wanted the page to look cool, if you look at how the wolf is positioning his leg: it's held up. Now, this is a stretch, but the panel makes it look like he is swinging his body around like a top in order to kick the human, meaning that his leg isn't held straight out in front of him, but straight out to his side. Again, this could be creative licensing on the webcomic creator's part, but there are ways to kick someone, knee kicks being one example actually.
So, this all boils down to the simple question again: is a digitigrade biped humanoid species capable of kicking someone or even kicking at all?
Please note, this question is not looking at whether digitigrade animals can kick, but how a biped, humanoid digitigrade species would be able a kick if they can, or if they can't, why can't they kick? Nor is the question looking at how hard a biped, humanoid digitigrade race can kick, and it doesn't matter if the species is mammalian, avian, insectoid, fungoid, etc. We are looking at whether a digitigrade, biped, humanoid species/race can kick at all, and if it can how? And if it can't kick, why can't it kick?