I am not sure of the situation described above, but at least with an ALPA Carrier, the union can take formal disciplinary procedures against a member (management pilots fall in this umbrella if they are on the seniority list, even if they aren't active). There is a very high bar for these disciplinary procedures. From the ALPA Constitution and By-Laws these charges can be brought for the following purposes...
It is very, very, very rare for discipline to be handed out, or even brought to this point, however, one of the disciplinary results can be to expel the person from the organization. That would open a host of problems for the individual as they could be in violation of the agency shop rules. That would in turn make the pilot not eligible to be on the seniority list, which in turn could put the pilot's career at the airline in jeopardy.
So the answer is 'yes', but would need to follow a very formal procedure. More than likely, the pilot group would just make it known that the individuals wouldn't be welcome on the picket line and they would hope they wouldn't fly, while the group is on strike.