In your "Suggest Link" template you have this area which (if you have link types switched on) allows people to choose what type of link to submit:

<IF 0>
<tr>
 <td class="labelscolumn"><span class="labels">Type of Link:</span></td>
 <td class="optionscolumn"><select name="type">{TYPEOPTIONS}</select></td>
</tr> 
</IF>

This may look slightly different, certainly if you're using the multilingual template set, but it's the section starting with <IF 0> and ending with </IF>.

If you want to force all links to be reciprocal, replace that section with

<input type="hidden" name="type" value="recip">

You can replace the same section in your "Edit Link" template, but needn't really bother since (by default at least) only administrators can change the link type.

You may also remove 'regular' from your link types setting, but doing so can force you to edit the 'regular' sections out of all your templates, so it's a waste of time.

If you're looking to ensure that people have properly reciprocated you can turn on the "check for reciprocal link on submission" switch.