Tough question. I assume electrical tilt through a RET is not an option either.
In effect, you want to decrease the cell edge coverage, without decreasing the cell core coverage.
Now there is two aspects to cell coverage : the DL, generated by the cell, and the UL, generated by users.
for the UL, it’s simple : prevent UE to be at cell edge. To achieve that, you must find a way to force the Handover and the Idle Mode Reselection to a neighbour cell. You can tune qRxLevmin / rxLevAccessMin, HO thresholds, TA thresholds, and so on, so that UE will not transmit signals to that serving cell.
for the DL, there’s no proper way to do it : you will need to decrease broadcast signal (CPICH, BCCH, RS) of your cell. In other words, you must decrease transmit power. That will decrease the cell core coverage as well (i’m thinking about “indoor”). Right now I’m not seeing any other way to do it. This method is not very effective, as you may be able to decrease the power (hence, the interference) by about 1dB to 3dB at most…
Also, look at features that improve interference resistance : repeated SACCH/FACCH, coverage extension, etc.