If both players have Boros Battleshaper, can I cancel out my opponent's ability with mine?
No. If you tried, then the creature would have both “blocks this combat if able” and “can’t block this combat”. The latter would win. First off, the first one says “if able”, and it isn’t able. A tapped creature wouldn’t be able to block for the same reason.
Second, even if it didn’t say “if able”, it wouldn’t matter because “can’t” always wins.


The Magic Golden Rules


101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.