The verb from the noun collection is collect, there is demonstration and demonstrate, reduction and reduce ... so injunction and ...?
Although rarer than the noun injunction, there is a verb 'to injunct' and it has appeared in some newspaper reports recently eg 'Has the rush to injunct gone too far ...?' asks this Guardian article (about half-way through) and '... Sir Fred Goodwin [...] had no right to injunct information ...' in this FT piece (third paragraph).
The OED describes the verb 'injunct' as colloquial. It is first attested only in the late 19th century, so injunct is a newer word than the noun injunction. The OED definition is 'to prohibit or restrain by injunction'.
Injunction, on the other hand, is first attested in the early 16th century. The first definition given in the OED is 'the act of enjoining' and, indeed, the noun 'injunction' comes from the verb 'to enjoin' (= to join together), which is a couple of centuries older. The legal sense, which we use today, namely a restraining order, was also in use in the 16th century.
The term for the situation when the longer word comes first, and then loses a suffix to become a shorter word, is 'back-formation'. Burgle is a back-formation from burglar, editor is a back-formation from edit and pea is a back-formation from pea.