Improve security of reflected code

Firstly can I just clarify that I am aware that the code outlined below is bad. Can I also reassure you that this is not for a real life product.

I have a VB.NET application which receives a string of math from the user, this string then has all the full stops removed and is then iserted into the middle of a resource string.

Together these strings form some VB.NET source code which is then compiled using a VBCodeProvider and called:

Dim compilerParameters As New System.CodeDom.Compiler.CompilerParameters With
{.GenerateExecutable = False,
 .GenerateInMemory = True}
compilerParameters.ReferencedAssemblies.Add(System.Windows.Forms.Application.ExecutablePath)
Dim assemblyInstance = (New Microsoft.VisualBasic.VBCodeProvider).CompileAssemblyFromSource(compilerParameters, code).CompiledAssembly.CreateInstance("FunctionMadeFromUserInput")
assemblyInstance.GetType.GetMethod("Evaluate").Invoke(assemblyInstance, {Input})

I would like your advice on ways to prevent user injection of malicious code, my specific efforts so far have been:

1. The only reference in the compiled assembly is to the current application (no reference to System etc.)
2. The users input is stripped of '.'s before compilation

While doing research to try to answer this question I have come across the System.CodeDom.Compiler.CompilerParameters.Evidence property however in .NET 4 this is deprecated. Then I discovered the idea of placing various <Assembly: System.Security...> attributes in the resource string however I am unable to work out which attributes will reduce the attack surface most effectively.

I am interested in preventing the user entered code from accessing disk and network IO and as many other potentially harmful things you can protect against.

Final disclaimer: I know this is not the best method of evaluating math, the math is not what I'm interested in. I am interested in ways of making execution of user input marginally less abominable.