As Sheldon implied, they are clearly looking for covering your country with their antennas. Therefore, whatever action you take, they will eventually counter it. GSM naturally provides algorithm to extend coverage, not to block coverage from others 🙂
Did you check which NCC they are using ?
To minimize their coverage, set your RXLEV ACCESS MIN to the minimum value (-110dBm), and if interference are globally bad, you might want to try SFH 1×1. I don’t have other tricks… On NSS side, remove them from the list of friendly PLMNs, so that your subscribers won’t camp on their network so easily. But that would take effect only on new SIM cards.
High level (or legal) actions are the only ones that will really make a difference. Get your CEO to talk to their CEO, and let them work it out.
Also, BTS are highly sensitive to electro-magnetic storms and solar flares. But unless you plan to mutate your DNA, thatprobably doesn’t help… 🙂