Medieval Latin *dēvetu- 'obstacle, prohibition' (updated)

Italian divieto 'prohibition', Old Spanish deviedo 'reserve (of land); prohibition, ecclesiastic censorship' and Basque debetü (Z) 'illicit', debeku (G, HN, Bazt, L, LN) 'prohibition' derive from Medieval Latin *dēvetu- 'obstacle, prohibition'¹, the participle of the verb dēvetāre 'to forbid, to impede'² (non attested in Classical Latin), usually regarded as a compound from the prefix *dē- 'from, away' and vetāre 'to forbid'³. 

The Latin verb is cognate to Celtic *wet- 'to speak' (Old Welsh guetid, Middle Welsh dyweddy 'speaks'), from which developped the form *woto- (Middle Welsh gwadu 'to deny', Old Breton guad 'denial')'⁴. From these forms and Hittite uttar 'word', Indo-Europeanists reconstruct an IE root *wet(H2)- 'to say, to forbid'⁵.

However, in Romance the meaning 'to impede' is usually represented by Latin dēfendere 'to move away; to reject'⁶, whose feminine participle is dēfēnsa- (Catalan devesa, Spanish dehesa 'meadow, pasture (usually fenced)').
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¹ The meaning 'obstacle' is reflected in the Basque compound gernu-debeku (Bazt, L) 'urine retention', whose first member is gernu 'urine'. 
² The corresponding Basque forms are debetatü (Z), debekatu (G, HN, Bazt, L, LN), debekau (B), bedekatu (B), bedekau (B) 'to forbid', for which Rohlfs proposed an etymology from *impedicāre 'to impede', which I consider semantically inadequate. 
³ Reflected in Spanish vedar 'to forbid, to impede', whose participle vedado 'reserve (of land)' has replaced deviedo in the modern language.  

⁴ R. Matasović (2009): Etymological Dictionary of Proto-Celtic. p. 418.

⁵ Similar to *wed- 'to raise's one voice', attested in Germanic, Balto-Slavic, Tocharian, Greek and Indic.
⁶ Which in some languages (e.g.  French défendre) also developped the meaning 'to forbid'.