I have that, people subbing for a day or so then unsubbing.
Those people can flutter but what annoys me is " OMG I LOVE YOU! I just subbed to you check out my channel!" But they never subbed to me at all, and I take a look at their channel and they have 1.5ksubs just from doing that and never subbing to anyone.
It took me a little bit but I've learnt to check my subs and only sub to people I genuinely enjoy and not because someone begged me to.