If you have assets with transactions but currently hold zero or very little of that cryptocurrency, don't remove them from your portfolio! That will delete all transactions as well and it will be as if they were never added.
Instead if you would like to hide assets with only exactly 0.00 holdings, enable the 'Hide 0 holdings' option under Settings -> Other Options.
If you would like to hide assets with small holdings, enable the 'Hide small holdings' option under Settings -> Other Options. At the moment, this hides holdings which are worth less than 0.2% of your total portfolio. We may make this option more customizable in the future.