Hi MouBoum,

For this you have to use access right. To achieve this you can add a csv for that group with no delete permission, you can do it manually or by a custom module.

To do it manually activate the developer mode then go to, settings -> technical -> security -> access control list , then search the model res.partner then check whether your group have access right in that model, if so edit and change the permission such that delete permission is unticked . if not add a new csv for that group without delete permission.

By code you can add it like this,

id,name,model_id:id,group_id:id,perm_read,perm_write,perm_create,perm_unlink
view_res_partner,access_right_name,base.model_res_partner,group_name,1,1,1,0