I've said no after initially offering someone my help learning German. The reason was that the other person didn't speak a language I was interested in (at the time, French or Italian). I felt that I needed to focus on my own language learning, too, and wasn't getting anything out of the conversations. So I stopped replying.
What I've also found is that, at the beginning, I collected many skype names from people who said they wanted to talk; but then we never spoke again. So I changed my strategy: I told people straight away what I wanted (i.e., 1 hour of conversation every week) and asked to set a time immediately. That worked because those people who weren't committed to learning usually didn't reply again and only those who were serious did. Some people want to speak "spontaneously" but, in my experience, that doesn't work.