I do not know if it would be possible but could you add a feature to exclude different coins on the multi port. We would check a box and when the pool switched coins if it was going to an excluded coin it would stay on the last coin or move to the next most profitable.
I like the idea, but unfortunately it isn't possible without redesigning our entire stratum architecture from scratch.
One thing to note is that not all coins for an algorithm are enabled on its' multiport. I hand select which coins are enabled for multiport mining, and take a variety of factors into consideration including buy support, price trends, average difficulty, etc. For example, there's not much value in mining a coin that has a declining price trend and takes a long time for blocks to mature, or a coin with high difficulty since switching on/off of it will likely produce very few blocks.
(Note that most multipools do not do perform the same level of research and hand selection of coins for multiports - its all about a block's profitability for them, which is problematic since the price of a coin at the time of mining and time of selling typically varies substantially.)
If there is a particular coin you're trying to avoid right now, I'm happy to consider removing it from multiport mining if there's a good reason.