They are non-reusable in the sense that if you take them out, you have to replace them. Valve guides are reusable until they are worn out; you don't replace them unless they are worn beyond usefulness. When they wear out, you can usually do one of three things: knurl; valve guide sleeve; new valve guide. You check the valve guide by the amount of side play there is in the valve when off-seat. The maximum amount allowed is going to be different for every engine. To understand the differences between the different fixes:
Knurl - Think of the handle of a ratchet where there is a cross hatch printed into it. This is the same thing for knurling a valve guide, only it's done on the inside where the valve stem rides. They do this to the inside and it squishes the portion of the valve guide out and replaces the space where material has been worn away. This is the cheapest and easiest fix, but unfortunately also lasts the least amount of time.
Valve Guide Insert - The original valve guide is left in place. The inside is reamed out just slightly and a new piece of valve guide material is driven into place. The new piece is then reamed out to size, which not only sets the size, but fixes it into place. This is a fairly cost effective way to fix the issue, with a minimal amount of effort to accomplish. This fix will last a long time (almost as good as a new valve guide).
New Valve Guide - The original valve guide is forced out of the head and a new replacement is put in its place. This is the best fix, but is more costly than the other two fixes. Since it is a new valve guide, it will last as long as ... well ... a new valve guide.
With all three fixes, you need to have a valve job done to ensure all of the valves and seats will seal correctly.