Hey Dan,
What you need to do is set up a Lookup instead of using the dimension to adjust the height. I have attached a dwg using your block as an example, but here is how you would do it. Insert a lookup and add the dimension to the lookup. Set up the heights you want and then label them in the lookup. Then the lookup is done. Now go into the properties of the dimension and set the 'Dyst type' to 'none' and set the 'Number of Grips' to '0'. Now the block will be locked to those 2 heights. If you wanted to still make it adjustable just set the 'Number of Grips' to '1' and you get your 2 heights and the ability to stretch it more if you want too.
Hope this helps,