Anything can be misused. A pencil can be misused by sticking it somewhere it shouldn't. It may even be possible to use the graphite (the part of the pencil one writes with) to conduct electricity, perhaps to electrocute someone.
So yes, one can misuse calcium by making someone overdose on it. By dumping tons of it in a lake to change the composition, perhaps combining all the CO2 with calcium to make calcium carbonate. By melting it down then catapulting it into a crowd... Anything can be misused.
The question you may want to ask instead, is: "Is calcium a dangerous element? That is, can it easily be misused?" The answer to that, is that that is unlikely. If it's just dumped somewhere, it will 'decompose' into common compounds like calcium carbonate and likely other compounds which aren't of particular danger to life. Calcium is commonly found, and needed, in the human body. It's taken as a vitamin supplement, by humans, in some form.
Saying that, though, finding even a common element in an uncommon form, such as pure calcium, can certainly have negative effects. It's unnatural. Despite that, I don't recall ever seeing or hearing any warnings about it nor did I see any such thing on it's wikipedia page.