Restricting Sales Org through Authority Check or Adivce

Urgent, My problem is that , i do have two user from two different regions USA1(USA) & MEXI(MEXICO), If USA1 user log into the system he should able to see Only his SALES Org (USA1/2/3)Or He can Change ONLY his Sales org to USA1 / USA2 /USA 3,if Mexico User log into the system He can Change ONLY his Sales org to MEX1 / MEX2 /MEX3. USA User can see sales organization of MEX1 but he cannot change any mexico Sales Org AND vice versa.

Can you please advice me on this issue OR if you are having any code ,please send me.

any help would be appreciated and Prompt Points will be assigned.