"When I see a new block with transactions that I didn't see broadcast previously, mark those transactions as suspicious. If I see double-spends of those transactions, stop building on that block-- assume it is cheating. Switch to the previous block (or alternate block if there's a block race going on)."
There are no incentives for doing that. If 98% of the network "discourages" a block, then those miners have a small chance of losing their blocks to the 2% that does not discourage the block. However, not discouraging a block has no penalty at all.