I too had this problem. I linked Alexa to Hue, then decided to go with ST only. I removed the skill from Alexa but it still would find the bulbs after I did a forget all.
I found out that Philips Hue will keep the Alexa skills authorization forever unless you do some work to revoke it. The Reddit post post below is where I started. Essentially you have to debug the Hue Bridge and manually revoke authorizations.
I ended up revoking all, including the ‘virtual device’ you add to get the api key, and starting over. Once I revoked the Alexa skill on the Hue Bridge I was able to forget the Hue bulbs on the Alexa web interface and they stayed deleted. I then authorized my phone and ST again.