to turn off rollups via mouse-wheel:

• open Settings -> Settings Editor
• on left pane select xfwm4
• on right pane expand general
• scroll down to mousewheel_rollup and UNCHECK 'Locked' and 'Value' checkboxes

please do confirm that this works OK for you. i am using Linux Mint XFCE 24, and this has worked fine for me from Mint 16 onwards.