There is currently no option for downvoting (flagging) using Musing.io. The Steem blockchain does still allow it, but it must be done from a user interface (UI) that permits flagging.
At the top of any post there is a flag on the right side. You can click this to flag (downvote) the post. If you have the requisite SP you will be able to determine what percentage vote weight you want to give the flag. Currently around 500 SP is required to get the voting slider bar on Steemit.com
On Busy.org click the ellipsis to get additional options. One of those options is to Flag the post. Busy offers the slider bar before 500 SP, so if you want to express your displeasure of a post without using a full-weight vote, Busy can be a useful option if you have less than the required SP.
You will need to authenticate this, but you can just scroll down to the bottom of your SteemWorld page and click on "Tools." Select "Manual Vote" and then fill in the link and other pertinent information. Change the type of vote to "Downvote (flag) and select the weight.
As you can see there are various options for downvoting. Some user interfaces allow it and even make it easy; others do not.
Before you can carried away with flagging though, I would offer a word of caution. Flags can be useful to express your disagreement with someone, but a comment can also do the job. Flagging someone can impact their rewards, and when money gets involved, people can take things very personally. If you begin flagging, you may have to prepare to be flagged back... even flagged repeatedly. Be careful with your criticism and flagging as it could end up being a costly lesson. Sometimes it's better to just walk away in peace.
tl;dr there are plenty of sites that allow you to flag, but flagging someone could lead to retaliation. Flag at your own risk.
Images are screenshots from Steemit.com, Busy.org, and Steemworld.org respectfully.