What does the word 「すまさんぞ」mean?

Question

Here is the full sentence. An old man talking himself with this one about a boy who should have been staying at home but he should not.

かえってきやがったら ただではすまさんぞ。

I'm not sure if it is conjugated from 済ます in a specific spoken dialect.

Answer 1

済ませる= ”I will let the matter pass” 済まさない= ”I will not let the matter pass”

すまさんぞ (済まさんぞ)= ”I will NEVER let the matter pass”