Well, if your suspended member is not logged in, how will the forums know he is suspended? The forum won't. He has to actually login for the forum to check to see if he's suspended.
It will treat him like any other guest or non-logged in user while he is not logged in since it doesn't know it's him. It only knows its him if he is logged on.
That's why it really doesn't matter if a suspended member can log in or not. They can look just like any other guest, they just can't do anything a guest couldn't do.
If you hide forums from guests, a suspended member won't see it either. If guests can't post, then your suspended member can't either.
So, in other words, if you implement the solution above, as soon as your suspended member clicked on "logout" they would be able to see everything a guest would see. The forum would think he's a guest.
Now, you could ban his IP address is he is especially troubling. But banning an IP address may ban more than just him, and may not stop him at all. Especially if he uses AOL or DSL or dial-up or cable modem.
If he isn't posting nasty things and is just looking at your site once and awhile, quiet frankly, I wouldn't worry about it. Maybe he is checking back to see if you un-suspended him yet.