How does one tovel a stainless steel food thermos that has a sticker on the outside that has powerful glue, and cannot be removed. As the food only makes contact with the inside, is this toveling acceptable?
If you aren't going to ever take it off because it can't come off, then it's OK. See Shulchan Arukh YD 120:13
The general issue with stickers is one of chatzitzah, that there is something (here a sticker) separating between the container (thermos) and the water of the mikvah.
This is only an issue if the separating object (sticker) is (1) covering the majority of the container, or (2) something that one would usually take off the container (makpid). (Biblical law is more lenient; see Eiruvin 4b.) Many (Rama YD 198:1) are stringent even if it is neither the majority nor something one would ordinarily take off, but this either does not apply to tevilas keilim or the case of a sticker is considered part of the container itself (as per Shulchan Arukh above; see also YD 198:17).
If you are going to take it off eventually, or would take it off before an important guest comes to your house, it is more complicated, and you should ask your local rabbi. (This is the issue of makpid lif'amim; see again YD 198:1.)
Usually even glue will come off if you use nail-polish remover (acetone).