I'm using Xubuntu 18.04 LTS, but I assume it will be the same solution:
All settings > Window Manager Tweaks > Accessibility > 'Key used to grab and move windows'.
I can't replicate your problem on my machine (Kubuntu 19.04 + KDE backports PPA), but here are some troubleshooting ideas:
Make sure you pressed the Apply button at the button of the setting window.
I assume you are using this IDE on the remote machine, but you haven't mentioned whether you you tried to disable Alt+Drag on the remote machine or the local machine, only "it starts to drag [the] window". But it's not clear whether this is the VNC window or the IDE window. Make sure you have changed the setting on the correct machine. You may very well need to do it on both.
I'm using Xubuntu 18.04 LTS, but I assume it will be the same solution: All settings > Window Manager Tweaks > Accessibility > 'Key used to grab and move windows'.
FYI Metakey could have been replaced the classical alt key
I can't replicate your problem on my machine (Kubuntu 19.04 + KDE backports PPA), but here are some troubleshooting ideas:
Apply
button at the button of the setting window.