I think that he has a point on the behaviour of some cyclists on shared footpaths.
However, advocating violence even in a tongue-in-cheek way is unacceptable in a national publication, as there is no guarrantee that the reader will not take this seriously and that it won't put the idea of actually implementing this into their otherwise vacant minds.
When you're writing for the Daily Mail, it should be assumed that at least 87% of the audience have never heard of a "joke" and everything should be printed in a simple and straightforward manner, with plenty of pictures, in the largest font possible.