to remove that message, you first need to uninstall it and then remove all entries from registry (dangerous part so do it only if you know it or can take a risk of the system going kaput requiring reinstall)
You get this message when you install pirated version of IDM as it constantly checks the license key daily. Once IDM server detects it the user gets the above message. Your best option is to Buy it or use other FREE alternatives.
Or You can group buy from IDM as they give heavy discount and LifeTime License for 10+ users
We bought the group license long time back and it still works Today
In a legit way go by all of the above suggestions or if you only like the way by which you got IDM in the first place just don't download any random torrent. There are forums for such softwares which have torrents with private trackers and is free from such nag screen