The word "hon-tou" = 本当(ほん-とう) may be considered as a noun or predicate, which has a meaning of real and right (as suggested by the other respondent). In terms of adjective usage, it is grouped as a na(な)-form adjective, and thus, when used as an adverb (meaning "really"), it is generally written as "hon-tou-ni" = 本当に(ほんとう に) .
In my personal point of view, however, "hon-tou (ni)" is an adjective/adverb to describe something positive. Hence, for the term "da-me" = だめ, which is a negative term, I would say that they do not well match together. If you want to say "really No", then I would suggest to use "ze-t-tai" = 絶対(ぜったい) instead, i.e., 絶対(に) だめ!
On the other hand, for the term "samu-i" = 寒い(さむい), you may use "tottemo" = とっても, meaning "very (much)" to emphasize the coldness. Hence, it can be said, 今は とっても 寒い, meaning "Now it is very, very cold".
ciao ciao / じゃね。