My baby won't let go of her wooden block! She's held it the whole morning and cries when I take it. Will clutching it for such a long period of time cause harm to her hand/joints?

I don't think it will affect her joints. Forcing her to let it go when she doesn't want to will make her grip on the block stronger and that might cause some stress. Why don't you wait for her to lose interest in the block or distract her with something else? Or just wait for her to take a nap or fall asleep ☺