Okay, I see what you are trying to do. What you could do is - whenever a player places a tile, test to see if that tile is of type "blah" and if it is, decrease Count to zero.
Now, whenever they want to place a start tile you can just check Count and see if it is zero or not. I'd still let them attempt to place a start tile if one already exists though. Why? Well, it would prevent them from having to find (and delete) the old tile first. I'd run a test whenever they attempt to lay a start tile, and if count = 1 then you could pop up a menu box that says something like "A start point already exists at (X:32, Y:57). Would you like to replace that with the tile you are attempting to place?" and if they select yes, then you let them place it and delete the old one, and if not do nothing.
Now, whenever they want to place a start tile you can just check Count and see if it is zero or not. I'd still let them attempt to place a start tile if one already exists though. Why? Well, it would prevent them from having to find (and delete) the old tile first. I'd run a test whenever they attempt to lay a start tile, and if count = 1 then you could pop up a menu box that says something like "A start point already exists at (X:32, Y:57). Would you like to replace that with the tile you are attempting to place?" and if they select yes, then you let them place it and delete the old one, and if not do nothing.