[Owncloud] How to delete only the address book of an user

[kfx]

Hi list,

- owncloud 5.0.11, mysql backend on Centos 6.4 x86_64

I'm facing a problem with a user with a messed up address book. He want 
to start over with a fresh address book but cannot delete the messed 
ones (The web UI say that he has not enough permission to do so).
In oc_contacts_addressbooks tables I can see a row with his login, will 
it be sufficient to delete this row ? I want to just delete the current 
address book not the rest (files, calendar etc.) so he can start with a 
fresh one.
Also I have in the same table some strange userid like 
c9b81a73-79f5-1632-0298-bf6794489352, should I delete them also ?

Thanks,
kfx