The problem
I see people with signatures that contain images that are within the height limitation but do have a scrollbar because they also add text to it. And the mods seem to allow this...

Now you could be wondering why does it matter. It matters to me because of the fact that when I'm scrolling down I get stuck in one of those signatures sometimes. And what is the reason to limit the height when people can go over like this? Then we can just as well remove that limit.

Even more amazing is that when I reported one of them, the mod that looked into it made it even worse by adding even more text to it...

The solution
Make the signature area a bit bigger, enough to hold an image of 200px and one or two lines of normal-sized text without getting a scrollbar (but keep the 200pxl limit rule).

or

Change the rule so that if you have a scrollbar your signature is oversized and will be coded.

I have that scrolling problem too sometimes. The solution is simple enough though; just move your mouse over the browser's scroll bar. 2-3 extra lines of text are allowed in signatures and even a few pixels over the height and width limit are allowed. Like you said, however, the signature area is set to exactly 200px in height. I love your first suggestion. Setting the sig. area to 205px or 210px should be sufficient for a couple lines of text.

Isn't the problem solved by moving your mouse to the side when scrolling? - I scroll down the page with my mouse lined up with the line between the post and user-information to the left... though that's also in part because it looks 'tidy'.

Increasing the height before it turns into a scrolling-thing wouldn't help since it'd simply make shorter signatures even longer too, overall increasing the amount of scrolling you have to do which seems to defeat the object.

Saying that if you have a scrollbar, it's oversized really isn't clear-cut - my signature has no scrollbar in Firefox, a ~2px scroll in Opera and a ~15px scroll in Chrome. Is it allowed because it's fine in FF or disallowed because some people use Chrome?

I'm using Firefox and your signature has a scrollbar for me, so yeah that wouldn't really work as a rule.

Although it would expand the overall page height, it would decrease the signature area and mean those who are over the limit by an allowed few px or have a couple lines of text wouldn't unnecessarily scroll. Take my signature for example. I never liked that part of the rules because it wasn't accurate in relation to what's really breaking the rules. As for people who believe they could get away with a 210px height instead of the allowed minute over size, I think I already take good care of that.
I'n FF 3.5.2 you have both a vertical and horizontal scrollbar.

quite an interesting topic this one is ive noticed that even if i dont post much

I really like your first suggestion.
Although, as far as I know, the mods do seem to allow this already, but it could only help to clear this up.