I've used this forum thread to "fix" my media buttons every time Logitech's SetPoint software updates, and the method to fix it has changed enough by 2018 that I thought I'd post an update.
First, the file is now in %userprofile%\AppData\Roaming\Logitech\SetPoint\ and is called user.xml.
As always, back up this file first! Then exit the SetPoint software before modifying the file.
Next, it won't work to remove the sections any more, you have to change their HandlerSet values from MM WHATEVER to GenericButton.
I just searched for HandlerSet="MM and changed the values as described above. For example:
<Button Number="786613" Name="Button">
<Param IconLoc="" Type=""/>
<Trigger Class="ButtonPress">
<Param Button="786613" EventType="33554432" FirstRepeatDelay="0" RepeatDelay="0" Silent="0" Type="0"/>
<TriggerState Name="ButtonDownUp" HandlerSet="MM NEXT_FF">
<Handler Class="MMPlayer2">
<Param Action="NEXT" LongPressTime="600" OSDImage="next.png" OSDText="Next"/>
</Handler>
</TriggerState>
</Trigger>
</Button>
Becomes:
<Button Number="786613" Name="Button">
<Param IconLoc="" Type=""/>
<Trigger Class="ButtonPress">
<Param Button="786613" EventType="33554432" FirstRepeatDelay="0" RepeatDelay="0" Silent="0" Type="0"/>
<TriggerState Name="ButtonDownUp" HandlerSet="GenericButton">
<Handler Class="MMPlayer2">
<Param Action="NEXT" LongPressTime="600" OSDImage="next.png" OSDText="Next"/>
</Handler>
</TriggerState>
</Trigger>
</Button>