When multiple AF points light up such as in your example, it means the camera interprets the information from its AF system to indicate all of them are within the acceptable depth of field. With a maximum aperture of f/3.5, shallow depth of field is not your problem.
But, since you asked, you can just look through the viewfinder at where you plan to be in the frame and select the single focus point that corresponds to that position. The Nikon D7000 allows for using a single selected focus point. Please note that designating it as a manually selected focus point doesn't mean you must manually focus to use it - it just means that you get to manually tell the camera in advance which focus point to use for autofocus! The instructions on how to select a specific focus point are located on page 96 of your D7000 User's Manual
Please see this answer for more on how the area of each focus point is larger than the little square in your viewfinder as well as why the far left and right AF points might not be as accurate as the center one(s). Although the camera is a different model from a different manufacturer, the concept is shared across brand and model lines in cameras with a large number of AF points. Nikon does tend to not include as many cross-type AF points as Canon, so it is more likely that half of your outer AF points are horizontally sensitive and half are vertically sensitive, rather than all cross-type as in the example at the link.
I think you will ultimately find that at least some of the blurriness you are blaming on AF is more a result of shooting with that lens wide open, at a shutter speed that can be particularly affected by mirror movement vibrations, and in poor lighting that is a different color temperature than what you've set your camera.
- Stopping down will work wonders for a lens such as your 18-200mm f/3.5-5.6. Those type of lenses are never that great, but they're usually not even good when the aperture is wide open. Of course stopping down means even less of the limited light you are shooting in will reach the sensor so it is time to think about adding your own light to the scene. Welcome to the world of portrait photography, where most all of that beautiful, natural looking light is totally artificial!
- Shutter speeds in the 1/80 second to about 1 second range are particularly susceptible to the vibrations caused by mirror slap. Any shorter and the exposure is over before the vibrations reach the critical parts of the optical path: the lens and the sensor. Any longer and the percentage of the exposure affected by the vibrations before they dissipate is low enough to reduce their significance in much the same way that you can take an extremely long exposure of a street scene and none of the people walking through are visible in your photo because they didn't stay in one spot long enough. Use mirror lockup with shutter speeds in the 1 second to 1/100 second range when the camera is properly stabilized to eliminate these vibrations.
- Selecting the correct white balance for the light will give better contrast and make your photos appear better focused. To test this shoot a few raw frames and then use a raw conversion application to look critically at the perceived sharpness as you change the WB to match or mismatch the shooting conditions. Look closely at the lines between darker and lighter areas in the frame (local contrast) and how they are affected by an incorrect white balance the over saturates at least one of the color channels. Overexposing a low contrast scene in subdued lighting can also have the same effect as rather than getting a few locally blown highlights as you would in a higher contrast scene, you see more general blooming all over the frame and the brighter spots bleed onto the adjacent areas that are dimmer.
