This app was mentioned in 11 comments, with an average of 2.82 upvotes
I never tried it, but this might work.
https://play.google.com/store/apps/details?id=com.arcai.netcut
Maybe, he will spend the night troubleshooting the problem instead of spending time with you.
If you are rooted you can use this app. With free version you can only disable/enable anyone's connection on the network but with paid version you can also control their connection speed.
netcut for android can do this. it require root phone . Filter MAC address and limit anyone internet's speed . exact match
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https://play.google.com/store/apps/details?id=com.arcai.netcut&hl=en_CA
Disable wifi for other devices connected to your router.
XDA link no longer works. According to Techno Luck's YouTube video comment, WiFiKill doesn't work with Android 5 and above. Anyway, he offers a link to WiFiKill Pro in his comments section. The download link requires care to avoid unintended downloads.
He also points to another video that recommends the NetCut app as a substitute for those with WiFiKill incompatibility. NetCut is available at the Play Store. Both apps require root.
There might be more then fair of devices are actually using your network . try netcut. it can list all network users AND kick out unknown devices .
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If you have windows laptop: http://www.arcai.com/netCut/s/ .
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Android Root phone : https://play.google.com/store/apps/details?id=com.arcai.netcut&hl=fr_CA
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The same software is available as a app on Google play store https://play.google.com/store/apps/details?id=com.arcai.netcut