Dear, Pix and RASH!
There is no notion “PLMN reselection”. There is only new-PLMN selection procedure, that triggered by overall loss of previous PLMN coverage (no suitable cells in previous (i.e. home) PLMN).
After new-PLMN selection in different country PLMN,MS is staying in that PLMN until any suitable cell exists in it, even the home network is already available.
RASH, you can prevent this problem by preventing your subscriber to register in that PLMN, or by expanding home PLMN coverage.