So, to force a user to register without leaving him the choice to use a different currencies, you need to edit clientregister.tpl of your active theme removing following block of code:
{if $currencies}
<div class="form-group">
<label for="currency" class="col-sm-3 control-label">{$LANG.choosecurrency}</label>
<div class="col-sm-9">
<select id="currency" name="currency" class="col-xs-12 col-sm-3">
{foreach from=$currencies item=curr}
<option value="{$curr.id}"{if !$smarty.post.currency && $curr.default || $smarty.post.currency eq $curr.id } selected{/if}>{$curr.code}</option>
{/foreach}
</select>
</div>
</div>
{/if}
