All EU has to do to make the situation truly difficult is deny British boats (is there actually such a thing, btw, aren't they all foreign owned??) to land their catch at EU ports.
It's more precarious than that - and requires no action or decision from any European government or the EU.
As soon as the first EU boat is impounded, every Continental port, from Dieppe to Esbjerg, will be blockaded by EU fishing vessels, preventing all cross-Channel freight traffic. The Channel Tunnel will face delays as every single pallet of goods is checked by French Customs, assuming that the French even keep it open. Every British lorry will be delayed for days. Every British boat in a EU harbour will be boarded and inspected in minute detail.
British supermarket shelves would empty in a matter of hours. Stocks of medicines and medical equipment would be depleted within days. Stocks of materials and components would run out in a week or two, forcing factories to close and putting their employees out of work. If Nissan, Airbus and BMW (and thousands of other less prominent businesses) had any doubts about relocating, these doubts will have disappeared in days and the factories will have all shut by Easter.
Johnson can only get away with blaming the EU for so long. If the people ever wake up to the fact that they have been well and truly conned, he'll be in big trouble - although the most likely outcome is that he'll resign long before the real trouble breaks out, claiming that he's battled through the after-effects of Covid and is now retiring to an unnamed friend's sunny island to relax and recuperate having 'Got Brexit Done.'