diff --git a/lib/Db/AssignmentMapper.php b/lib/Db/AssignmentMapper.php
index f2731431f..0904cd770 100644
--- a/lib/Db/AssignmentMapper.php
+++ b/lib/Db/AssignmentMapper.php
@@ -92,6 +92,21 @@ class AssignmentMapper extends DeckMapper implements IPermissionMapper {
 		return $this->findEntities($qb);
 	}
 
+	public function deleteByParticipantOnBoard(string $participant, int $boardId, $type = Assignment::TYPE_USER) {
+		$qb = $this->db->getQueryBuilder();
+		$cardIdQuery = $this->db->getQueryBuilder();
+		$cardIdQuery->select('a.card_id')
+			->from('deck_assigned_users', 'a')
+			->innerJoin('a', 'deck_cards', 'c', 'c.id = a.card_id')
+			->innerJoin('c', 'deck_stacks', 's', 's.id = c.stack_id')
+			->where($cardIdQuery->expr()->eq('a.participant', $qb->createNamedParameter($participant, IQueryBuilder::PARAM_STR)))
+			->andWhere($cardIdQuery->expr()->eq('s.board_id', $qb->createNamedParameter($boardId, IQueryBuilder::PARAM_INT)))
+			->andWhere($cardIdQuery->expr()->eq('a.type', $qb->createNamedParameter($type, IQueryBuilder::PARAM_INT)));
+		$qb->delete('deck_assigned_users')
+			->where($qb->expr()->in('card_id', $qb->createFunction($cardIdQuery->getSQL()), IQueryBuilder::PARAM_INT_ARRAY));
+		$qb->executeStatement();
+	}
+
 
 	public function isOwner($userId, $id): bool {
 		return $this->cardMapper->isOwner($userId, $id);
diff --git a/lib/Service/BoardService.php b/lib/Service/BoardService.php
index 0d0f32796..d3d3260d0 100644
--- a/lib/Service/BoardService.php
+++ b/lib/Service/BoardService.php
@@ -510,10 +510,7 @@ class BoardService {
 		$acl = $this->aclMapper->find($id);
 		$this->boardMapper->mapAcl($acl);
 		if ($acl->getType() === Acl::PERMISSION_TYPE_USER) {
-			$assignements = $this->assignedUsersMapper->findByParticipant($acl->getParticipant());
-			foreach ($assignements as $assignement) {
-				$this->assignedUsersMapper->delete($assignement);
-			}
+			$this->assignedUsersMapper->deleteByParticipantOnBoard($acl->getParticipant(), $acl->getBoardId());
 		}
 
 		$this->activityManager->triggerEvent(ActivityManager::DECK_OBJECT_BOARD, $acl, ActivityManager::SUBJECT_BOARD_UNSHARE);
diff --git a/tests/unit/Service/BoardServiceTest.php b/tests/unit/Service/BoardServiceTest.php
index bba051cc5..3ab99b0a7 100644
--- a/tests/unit/Service/BoardServiceTest.php
+++ b/tests/unit/Service/BoardServiceTest.php
@@ -448,10 +448,10 @@ class BoardServiceTest extends TestCase {
 		$assignment = new Assignment();
 		$assignment->setParticipant('admin');
 		$this->assignedUsersMapper->expects($this->once())
-			->method('findByParticipant')
-			->with('admin')
+			->method('deleteByParticipantOnBoard')
+			->with('admin', 123)
 			->willReturn([$assignment]);
-		$this->assignedUsersMapper->expects($this->once())
+		$this->assignedUsersMapper->expects($this->never())
 			->method('delete')
 			->with($assignment);
 		$this->aclMapper->expects($this->once())