As Sleepygeckos said this is fairly common for them to do. My little ones seem to be much worse about it than the larger ones. They will paint the sides of their kritter keepers with the food.

Depending on how bad it is I use one of two methods:
1.) If it is just a little bit then I will wet their foot and then let them go once I know the food is starting to soften again and come off. I've been bit a few times using this method because I try to rub it off a little.
2.) Soak them with some damp paper towels for a little while. They will usually fire up when you use this method too as an added bonus.

I typically use the first method because I have a horrible habit of forgetting things so I end up soaking them much longer than needed. The second method is probably better though especially for beginners. You need to make sure to hold the gecko still without hurting them when you use the first method and that can be hard to do.