Can a will itself include a clause trying to prevent a dependant from making a support claim?

A testator can write a clause trying to stop a dependant from bringing a support claim, but it won't actually work — the right to apply for dependant support under Part V of the Succession Law Reform Act is a statutory protection, and a will can't override it with a "no contest" or similar clause aimed at eligible dependants. The whole point of Part V is to let the court step in when a will, or an intestacy, doesn't adequately provide for someone the deceased was supporting or legally obligated to support before death — allowing a clause to defeat that would undermine the reason the law exists.

That doesn't mean a will has no influence at all: what it actually leaves a dependant, and the testator's stated reasons, can still be relevant background the court considers in deciding whether adequate provision was made and what amount is appropriate. But a clause purporting to forbid or penalize a dependant for making a claim is not a way to shut the door on Part V. Anyone drafting a will with a blended family or complicated dependants should get advice on realistic ways to manage that risk, rather than relying on a clause like this.