An honest answer can arrive gently without becoming less definite.

Firmness and harshness get confused, mostly by people who have only ever seen the two arrive together. A clear no can be delivered warmly, with real regret and without a long apology, and still be a no. Softening the tone is kindness. Softening the content is a favour to nobody, since it leaves the other person guessing and hoping. Vagueness is often reluctance wearing good manners.

When you need to decline something this week, be warm in the first sentence and unambiguous in the second. Resist the urge to add maybe, which costs the other person more than the plain answer would.