Lower std.deviation of a distribution?

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How would one lower the std.dev => $\sigma$ for an arbitrary distribution?

The reason why I ask, is because I have a distribution which tend to get far above my threshold value, and i am running out of ideas on how to keep the value low.

The thing i been doing now is decreasing the population size, values which lie 2$\sigma$ away, but I cannot continue doing that.

what else can i do?