Can a player reach over the net to block in volleyball?

Yes, but only after the opponent has made their attack hit. A blocker may not touch the ball on the opponent's side before the attacker contacts it. Penetrating before the attack is a fault.

Full Explanation

Per FIVB Rule 14.3.1, a block action that crosses the net is permissible provided the blocker does not interfere with the opponent's play before the attack hit is made. Specifically, after the opponent has completed their attack hit, a blocker may reach over and beyond the net to complete the block. If the blocker's hands cross the net and touch the ball on the opponent's side before the opponent's contact, it is a fault resulting in a point for the opponent. Blockers must also avoid touching the net during the block action (FIVB Rule 11.3.1). These rules apply under FIVB, USAV, NFHS, and NCAA.

Rulesets covered: FIVB/USAV/NFHS/NCAA  |  Rule Reference: FIVB Rule 14.3.1; FIVB Rule 11.3.1