Ok so this is a tad late - I came across this searching for something else, as it didn't quite trip my keyword alerts - but better late than never. The short answer is that yes, there were plenty of people who were upset at the decision not to topple the Franco regime (on which you can read more here).

Given that your interest was sparked by reading of the various foreign volunteers in Spain, these same volunteers are a logical place to focus. When these foreigners were officially withdrawn from Spain in late 1938, during their farewell parade in Barcelona they promised that they would continue the struggle abroad. While they may have intended this to mean that they would raise money and political pressure to support the Republic's struggle - which many, though not all, certainly did - it became clear soon afterwards that the Republic was not going to emerge victorious from the civil war. By 1939, those who had returned to democratic countries had begun organising themselves into veterans' associations, such as the British International Brigade Association (IBA), with the explicit remit of campaigning for the restoration of a democratic Spain and overthrowing the Franco regime. These organisations were generally - certainly in the USA/UK cases - formally or informally associated with the Communist Party, who had often played the central role in organising and leading the volunteer contingents in Spain.

During the Second World War, actually undertaking this kind of struggle was difficult. Plenty of Spanish veterans had signed up for Round 2 of what they perceived to be the same struggle against European fascism, making activism more difficult. Political controversies could also sap their cohesion and spirit - efforts by communist-aligned members of the IBA to throw their support behind the USSR's invasion of Finland, for instance, backfired considerably in the face of widespread sympathy for the Finnish cause. On the issue of Spain, however, the ex-volunteers could speak with a respected voice, and were listened to by a broad spectrum of the left, and the IBA managed to continue to make private and public calls for the Franco regime to be a target for Allied military and diplomatic intervention.

Per the linked answer above, however, UK and to a lesser extent US diplomats were leery of actually taking effective action against Franco after he had been sufficiently bullied to scale back his support of Nazi Germany by c. 1944. Like many exiled Spanish Republicans - quite a few of whom had fought for the Allies - the ex-volunteers were deeply disappointed by the outcome of the Potsdam conference, and continued to campaign for the overthrow of the Franco regime and against its rehabilitation by the international community. I've seen some hints that a handful of ex-volunteers were involved in anti-Franco resistance activities inside Spain (this possibility is also suggested in Pan's Labyrinth, interestingly) in the late 1940s and early 1950s, but open resistance largely petered out in this period and I'm not sure that they achieved much (my own read of the sources is that their vagueness stemmed from a lack of concrete achievements rather than a desire to preserve secrecy). Veterans' organisations continued to campaign against the regime on issues such as human rights abuses, political prisoners and so on, with patchy success. Other foreigners were inspired to try and lend a hand - my favourite was the effort of a young (18!) Scottish anarchist named Stuart Christie, who crossed the Pyrenees, dressed in a kilt and armed with plastic explosives, with the intent of blowing Franco up. He made it as far as Madrid before being caught and sentenced to death, though this was eventually commuted and he served only a few years in jail before being released back to Britain. He is still alive today, last I heard, having had a long and varied activist career.

Needless to say, Franco's regime survived the foreign pressure that such individuals and groups were able to exert. Ex-volunteer associations had perhaps more success in shaping historical memory and narratives surrounding the civil war itself, building their own archives, collecting testimony and writing their own histories. Even though just about all the ex-volunteers are now dead, their influence in how the conflict is perceived is still impossible to ignore when writing this history today.