Can Kalman filter remove 50 Hz noise?
With your edit it becomes clear that you've modeled your problem incorrectly:

While the offending signal appears shortly with a frequency of 50 Hz, that is by no means the frequency content of the interference!

(also, your filter isn't well-designed, probably too short, judging from the impulse response it displays, to filter out 50 Hz)

You'll find that interference to look more or less like dirac impulses – and these have a very high bandwidth! So, in spectral domain (plot a PSD!) you'll notice that there's not only a peak at 50 Hz, but wideband interference.

Technically, you seem to have a comb of such, and that would actually lead to a line spectrum again, which you could "kill" with a multi-notch filter.
But then again, these aren't really diracs, and you'd need a very long filter to even come close to being able to notch these out.

I think a time-domain solution is much more promising than filtering: 

Simply build a threshold that detects when a burst interferer starts and ends, and zero out all samples in between.

If you want to be really elegant, you could try to use for example linear prediction to fill these gaps instead of zeroing, but my guess is that you'd only win a tiny degree of signal quality.