Ram,
I’m afraid there is not. Your network is certainly considered as a possible PLMN, as hard-coded in their SIM cards, so everytime their home PLMN is not reachable, they’ll try to attach to your network.
But frankly, I don’t see why that’s a problem… The LU success rate is not a KPI, it should be analyzed accordingly to your situation.
Unless it totally overloads your HLR, it doesn’t look like a problem…