The Flatspin is an inexpensive Chinese bike. While a shim should not be necessary, the manufacturing tolerances may make one so. Stem shims are not necessarily dangerous - in fact some stems come with them to use 1 1/8"stems on 1" steerers. Rubber, tape, or anything compressible is a bad idea. The classic low budget shim is a strip of coke can. You can cut it to size with scissors. If you need more than 1-2 thicknesses to get it tight I would say the stem may be streched, cracked or otherwise damaged.