、。
Voters must not be bribed.
Literal
Voters [topic-は], bribe-[passive-されて]-is-not-allowed-[formal-ならない].
買収する ('to bribe, buy off, acquire through purchase') is the suru-verb; the passive 買収される is 'to be bribed.' ~てはならない is the formal prohibitive (vs. ~てはいけない, which is more everyday). The whole sentence has the cadence of a civic or election-law declaration.