Disable Lead to contact by condition

[shalmaxb]

Is it possible to disable the lead to contact conversion by condition? In my lead record I will not want the lead converted, if not all fields, which I want in a contact, are filled. Conversion should be able only for complte records.

[shalmaxb]

Meanwhile I solved it by using status. There is status "Recycled", which disables the converting button. I change the status to Reycled, when a record is not complet. if it is completed I change again by formula to New.
Result as desired.