。
Don't throw balls at the fence.
Literal
Fence [at-に] ball [object-を] throw-against must-not.
投げつける is 'to throw (at a target)' — 投げる (throw) + つける (attach/against). ~てはいけない is 'must not' — a prohibition softer than な but firmer than ~ないでください. This level of prohibition is typical of rules and posted signs.