The harder but more effective protection of the two I thought of to suggest right now is requiring the Murder skill in order to harm the questgiver (chop tree or chip rock on them), as this signifigantly raises the cost and time investment for people who want to make alts to kill Questgivers. Maybe require murderous rage be active too to leave them more vulnerable to potential player punishment.
The easier but less effective one is for killing Questgivers to leave a murder scent and redhand the murderer. This would definitely be better in conjunction with the first part as in its own it doesn't fix the spawn naked alt into chopping random NPCs with zero investment or zero consequence.
I am aware this won't fix the problem wholly, but it will raise the expenses significantly thus lowering the amount of people doing it.