地主は不法占拠者を立ち退かせるために裁判を起こした。
The landowner filed a lawsuit to evict the squatters.
Loanword from English 'squatter'. 不法占拠者 is the native Japanese legal term, while スクワッター is more colloquial and often used in news about overseas squatting.
Compound of 不法 (illegal) + 占拠 (occupation) + 者 (person). A straightforward modern coinage for legal and social contexts.