Obstruction
Hindrance
If you thwart and stop something, you are an obstruction or hindrance.
そし
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
その証人が法廷で証言するのを、何としてでも阻止するんだ。
Do whatever it takes to prevent that witness from testifying in court.