Remmina doesn't eat keys

Remmina has this configurable in the 'Preferences' -> tab 'Keyboard' -> Field 'Grab keyboard'. On my Ubuntu installation this is the right Ctrl key.

Works for me and seems to behave like a toggle key. Pressing it again makes modifier keys working on the local machine again.

Depending on your client environment, using a keybinding to toggle the 'grab keyboard' might not work for a number of reasons.

As an alternative, you can use the 'grab all keyboard events' button in the toolbar when in fullscreen mode.