I mean, in such a way as to make it feasible to have a collectible sample of an element like astatine, francium, or protactinium (ignoring their chemical toxicity, which could probably be contained in the same way as that of arsenic, thallium, or lead). I've never heard anything about this (and of course I am aware of why these elements normally have no stable isotopes).
If a radioactive species decays solely by electron capture then completely ionizing the atom would remove the possibility of electron capture.
Storing the atom(s) in a very high speed cyclotron-type device would allow relativistic time dilation to extend the half-life without any limit.
You could prevent (or at least dramatically slow the rate of) beta decay by surrounding the radioactive nuclei with a dense gas of degenerate electrons. If the Fermi energy exceeds the maximum energy of the decay electron then beta decay would be blocked.
Such a process is thought to occur in nature in the crusts of neutron stars. Very exotic, neutron-rich nuclei can exist without decaying because of the surrounding dense, degenerate electron population.
Decay through weak nuclear interactions is affected by irradiating the nucleus with the appropriate stimulus, such as gamma rays of the appropriate frequency.
If the gamma ray is energetic enough it can excite a radioactive reaction. When applied to long-lived isomers of certain isotopes, this has been proposed as a "nuclear battery" energy source for interstellar spacecraft. A very recent article in *New Scientist" reports a claim that it has been demonstrated in the lab, although there are some critics of the claim.
If the gamma ray is less energetic it can still perturb an excited nucleus. Certainly with excited atomic states (electron orbitals), continual perturbations can delay the return to the ground state and accompanying emission of energy. This should theoretically apply to radioactive nuclei, but I do not know if that has been tested experimentally.
Similar effects with the strong nuclear force would require a source of gluons or something, which I do not think is even theoretically feasible.