Flag the activity to remove the signature extraction.
From my onboarding topic for newbies –
Tip for the future –
There will be times you generate a BT that you don’t want applied. For example, an abrupt stop at the line may generate a BT but you didn’t fail naturally and you could have kept going. Or you may have power spikes in the data which Xert attempts to filter out during analysis but one snuck by and bumped your signature. Or you got a Near BT which slightly lowers signature values and you prefer not to do that. Its not necessarily a bad thing as it improves your chances to generate a BT in the future but many hate to see any decline while they’re training. 
For these situations you can flag the activity which retains the strain score for TL purposes but skips signature calculation.
to 
When you’ve accumulated enough activities on file try riding a fitness test workout in Slope mode to establish a valid starting signature.