Dear Jean!
This is not true, because, if there are at least one suitable cell in the neighbouring country’s network and MS was registered in this network, then MS will don’t attempts to select its Home network. The Home PLMN searching timer is active only when MS was registered in HPLMN country.
You must to improve the coverage of your network (by any means) to avoid MS to camp on cell that belongs to foreign network.
To ParaHO:
In GSM SIM the VPLMN is added in Forbidden PLMN List only if “Plmn not allowed” message is recieved by an MS in responce to on Loc_Upd request from a VPLMN. No another way to doing this. Perhaps there is capability to update this list by HPLMN operator via OTA in UMTS SIM.