Thank u so much for your answer
Meanwhile I found a workaround: fortunately the BP involved are around twenty so it's possible to manage them one by one.
1_ When u try to modify or delete a minitemplate from template appear a pop-warning in which are listed all the entries (BP) that stop the save.
2_ In the WEB_UI it's no possible to delete the template, but it's possible to enter in "Sales area Data" and modify the data involved in the mini-template using the button "Own Data".
When u enter in "Sales area Data" near the button "Own Data" there is a notice:
"Data from Template 'Z_NAME OF YOUR TEMPLATE', Explicit Assignment to Business Partner"; after the modification this notice disappear.
This kind of modification cuts off the relationship between BP and template (so in the list above you have one less entry).
3_Repeat step "2" for all the BP listened in the pop-up warning and in the end you will be able to delete or modify mini-template.
Obviously, this method is possible to use only if you can manage the BP one by one. It can not be the solution to maintain a real custom base. It's just a workaround
regards
Simone