Difficult people where i volunteer, advice please

Hi all. I volunteer for the British Heart Foundation. I have been at my local one for 6 months now going in three days a week for about 4 hours each day. Well on Sundays there are a few community service guys. Today I was in and had a lot to do but a few times in the day two of them (one has annoyed me in the past but I stayed out of his way ever since) came along and I know this does sound silly but it really did annoy me. They just kept asking "how many hours have you got left" implying I am on community service as well. Then they mumbled other things while watching me get on with my tasks and giggling like school girls. I was actually having a good day because I was able to use the phones more as I have now been trained and getting my confidence up and then these two guys just randomly start pestering me. Most times I can deal with random comments from people but today it just annoyed me. I just need some advice on how to deal with them. Someone else volunteering there did suggest replying to them but in a jokey way. Example was when they asked "how many hours you got left then,." and replying to them "well I have X amount of hours because I did X to my family" because it may make them stop saying anything weird to me. The strange thing is one of those two guys finished their community service today and the other has never bothered me before. It is a bit like you get a bully and a sidekick and once the bully has gone the sidekick becomes mute and that is what happened. The bully left at 2pm and after that the sidekick never said a word to me.

Apart from the annoyance of the day I was told by some other volunteers that they noticed my confidence was a lot better today and I did really well using the phones. So I am trying to concentrate on the positives. Any advice on how I can deal with people like the above would be appreciated.