FVWM: How to disable keys on keyboard if xmodmap does not more work?

Hello *,

I have on my ThinkPad near the 4 arrow keys to other keys (Pg Back,
Pg Fwd) and xmodmap does not more work under Debian Stretch.

The problem is, if I am in Firefox and my webmail that I have hit
already many times the "pg back" button and then my whole EMail I
wrote is gone.

Does someone know, how to disable this two buttons?

xev giv me following:
----8<------------------------------------------------------------------
KeyPress event, serial 36, synthetic NO, window 0x4400001,
    root 0xb2, subw 0x0, time 737341228, (169,-6), root:(980,34),
    state 0x0, keycode 166 (keysym 0x1008ff26, XF86Back), same_screen
YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyRelease event, serial 36, synthetic NO, window 0x4400001,
    root 0xb2, subw 0x0, time 737341308, (169,-6), root:(980,34),
    state 0x0, keycode 166 (keysym 0x1008ff26, XF86Back), same_screen
YES,
    XLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyPress event, serial 36, synthetic NO, window 0x4400001,
    root 0xb2, subw 0x0, time 737351985, (169,-6), root:(980,34),
    state 0x0, keycode 167 (keysym 0x1008ff27, XF86Forward),
same_screen YES,
    XLookupString gives 0 bytes:
    XmbLookupString gives 0 bytes:
    XFilterEvent returns: False

KeyRelease event, serial 36, synthetic NO, window 0x4400001,
    root 0xb2, subw 0x0, time 737352057, (169,-6), root:(980,34),
    state 0x0, keycode 167 (keysym 0x1008ff27, XF86Forward),
same_screen YES,
    XLookupString gives 0 bytes:
    XFilterEvent returns: False
------------------------------------------------------------------------

and as I sayed, xmodmap does not work, nor my deadkeys for many estonian
special caracters.

Thanks in advance

-- 
Michelle Konzack        Miila ITSystems @ TDnet
GNU/Linux Developer     00372-54541400