Those who answer by email often cause messages to have a suffix phrase like this:
Sent from my iPhone
Sent from my Samsung Galaxy
and so on…
Is there a way to enter all such phrases in admin panel and let the engine auto-remove them?
NB. The phrases can be in different languages.
Going necro on this one - is there any progress on this? This is getting a problem on our Discourse instance, too.
Or another approach to automatically determine signatures?
Automatic, 
There is this, they spent three years building it
The dataset consisted of 2912 email lines. Out of 1030 signature lines 954 were classified correctly. Out of 1882 non-signature lines 147 were mistaken for signature. Overall it gives us 92% success rate and 78% area under the ROC curve. Which could be regarded as excellent and fair correspondingly.