Is there a way to decrease the rate of nuclear Beta decay?

Yes. Have a look:

“The ‘Reifenschweiler effect’ is the observation that the beta-decay of tritium half-life 12.5 years is delayed reversibly by about 25-30% when the isotope is absorbed in 15 nm titanium-clusters in a temperature window in between 160-275 C. Remarkably at 360 C the original radioactivity reappears. The effect is absent in bulk metal. Discovered around 1960/1962 at Philips Research Eindhoven, The Netherlands Reifenschweiler extensively discussed his observation with o.a Casimir (the director of research at the time), Kistemaker (ultracentrifuge expert), and although no satisfactory explanation was found, R. was allowed to publish it. At the time a unique example as to how an electronic environment might affect nuclear phenomena.”

Here is a speculation to explain the effect.

if the first link stays dead , here is an archive of it.

As there is very little on the web since 2011, and the main site is nor responding, it was probably a speculation and measurement problems, see here.

A special kind of decay, electron capture $\beta^+$ decay, by definition depends on the electron density of the environment, which may change with pressure, chemical composition etc. There's a Wikipedia article: http://en.wikipedia.org/wiki/Change_of_decay_rate

It is possible to increase the probability of the beta decay

For the $\beta^+$-decay of $^{22}\text{Na}$ in the metallic environment Pd cooled to T=12K the $^{22}\text{Na}$ half-life was observed to be shorter by $1.2\pm 0.2$%