A defense against this would be to mark the multisigs that are being used as anchors. Rather than 2 of 2, you could use 2 of 3 with the 3rd key being a standard value.
You can then set k based on how many trades are happening on the altcoin chain.
Attacker can make anchors for nearly free (large transaction values relative to transaction fees):
...but then an attacker can DE trade to himself to fool your algorithm into an unbounded value of k (as high as the attacker wants to make it).