I don't think you have a memory leak issue. An increase in memory usage does not necessarily mean a memory leak. Each time you click on the button, there may be many memory allocations and disposes that occur that are not part of your code but are part of the .NET Framework itself. The .NET Framework has a garbage collector function that periodically frees memory after it has been disposed. There also may be memory allocations and deallocations in the Windows APIs that the .NET Framework uses.
See Garbage Collection
Unless you can show that your PC runs out of memory after tens of thousands, hundreds of thousands or millions of button clicks, I'll continue to believe that you are just seeing the natural growth in memory usage inherent in the .NET Framework memory handling processes and Windows memory handling processes.