Limit the amount of current passing through a circuit to a set threshold?
Is there some kind of simple device i can put inline to limit the
  power coming from the aux input? Ideally it would be something that
  blocks all voltage above a certain threshold.


A simple attenuator with two resistors configured as a voltage divider will reduce the signal level and prevent overloading, but the sound will probably be muffled due to greatly accentuated bass and reduced treble. Why? The two graphs below (both scanned from National Semiconductor Audio Handbook 1978 edition) show the typical output of a tape head and frequency response of the associated tape pre-amp. 





The tape head uses a coil to read the signal off the magnetic tape. Since magnetically induced voltage is proportional to frequency, the tape head output has a steeply rising frequency response until dropping off when the magnetic domains on the tape become smaller than the head gap. To compensate for this the tape pre-amp has the opposite response, with a very high gain at low frequency sloping down to lower gain at high frequency.

When the tape head and pre-amp are combined the overall frequency response is relatively flat. But your phone output already has a flat frequency response, so an attenuator which reduces the level equally at all frequencies will cause high frequencies to be ~30dB down at the output of the pre-amp.        

For overall flat frequency response your attenuator needs to mimic the frequency response of the tape head, as well as reducing the signal level. This can be done by adding a capacitor in series, which creates a 'high pass filter' that progressively attenuates lower frequencies. We want this to start happening below ~2kHz, with a loss of ~30dB above it. 

30dB loss corresponds to a voltage divider ratio of \$\sqrt{\log_{10}^{-1}{(30/10)}} = ~31.6\$, which can be achieved with standard resistor values of eg. 10k&ohm; and 330&ohm;. The 'cutoff' frequency of an RC filter is \$fc = 1 / (2 \pi RC)\$. The input resistance of your voltage divider is 10.33k&ohm;, so the capacitance required for 2kHz is 1/(2&pi;*2000*10330) = ~7.7nF. Using the nearest standard value of 8.2nF the circuit looks like this:-  







simulate this circuit &ndash; Schematic created using CircuitLab

which produces this frequency response:-

      

A 'Hi-Fi' tape pre-amp will also boost higher frequencies above ~6kHz to compensate for tape head roll-off at the upper end. We can reproduce this characteristic by adding a capacitor across the attenuator output to form a low pass filter. With 47nF in parallel with R2 the response looks like this:- 



These values provide a starting point, but may not be the best for your amplifier. Feel free to change them if you find the level is too high or low or the frequency response doesn't suit your tastes.