How to disable assigning a value to a property?

[Jack Applegame]

Code:
```d
import std.stdio;

struct Field {
     void opAssign(int a) {
         writefln("Field.opAssign(%s)", a);
     }
}

struct Register {
     Field clock(int a) {
         writefln("Register.clock(%s)", a);
         return Field();
     }
}


void main() {
     Register register;
     register.clock(1) = 10; // works, good
     register.clock = 10;    // works too, how to disable it?
}
```
How to disable `register.clock = 10;` but allow 
`register.clock(1) = 10;`?
I want to get a compilation error on `register.clock = 10;`