I'm not sure I can. As with all the effects in the game, you should always be able to pay the cost, even if the ability fails to do anything or just does nothing. This is the case here, (limit 1) refers to playing the fortification from discard, and not to use of the activated ability, the same as in case of Trust Me As You Once Did the limit refers to Strength bonus, and not how many times you can use it.
You definitely can't, as there are situations when you need to be able to use it multiple times (for example to exhaust your ring-bearer before your opponents sets up a
Lost to the Goblins bomb).
But there is a problem with limits (especially in case when you have multiple copies of the card out, like
Sixth Level or
Shadowplay, or when you can use the limit partly and then try to use the remaining part, like
Sting). I think those two things would definitely improve gameplay:
- for each card with a limit, actual state of this limit's usage should be displayed in card info box (the game already records it so that it's correctly enforced),
- when the limit is 100% used (4 twilight from
Sting, +3 strength from
TMAYOD, 1 fort from
Sixth level etc.) a warning should appear also on the table (for example a red circle in the lower left corner of the card should do this job well).
BTW, a minor issue regarding limit (I remember this happening, but some time ago, so maybe it is already fixed). When I have Frodo with
Sting, 2 twilight in the pool and I exert Frodo using
Sting, revealing 4 orcs in opponent's hand, 2 twilight is removed (correct, as the pool can never go below 0), but IIRC 4 was removed from the limit (so that when I play some cards after removing, adding 2 twilight more, I can't exert Frodo again to remove it, I think I should be able to do this as
Sting has removed just 2 twilight yet, even though it tried to remove 4).