The best solution would be to downtilt or change the antenna orientation, in order to change your best server areas. I know it takes more energy to actually physically change the antenna design, but it really is the proper solution.
Any other solution would lead to a serious impact on all other areas of coverage, not only the building…
What you could do is increase the Cell Reselect Offset of cell B, so that the call is established in cell B, initially.
Then, make sure the call stays in cell B, by increasing the HO MARGIN (B, A).
But as I said above, those settings will disturb the rest of the cell too…